"Fall of the Leaning Tower"

PBS Airdate: October 5, 1999
Go to the companion Web site

____: During the following program, look for NOVA's Web markers which lead you to more information at our Website.

NARRATOR: In the 12th century, in a Europe ravaged by war, famine and disease, the rich Italian city of Pisa rose above the horror by erecting the most magnificent Tower the world had ever seen. 800 years later this bold architectural dream has produced a nightmare. Inch by inch, year by year, the Tower is slowly tipping over. What was once an amusing tourist attraction is now a threat. The top hangs 17 feet south of the base - and the Leaning Tower of Pisa is on the brink of disaster.

JOHN BURLAND: A collapse might be triggered by a storm or an earthquake.

JANE MORLEY: I'm astonished that it's still standing.

NARRATOR: Pisa is shaken by several tremors each year. If an earthquake rocks the ground, or a storm sweeps in from the nearby coast, one of the wonders of the world will be lost forever. Can modern ingenuity prevent this nightmare and save Pisa?

____: Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.

____: CNET, bringing the digital age into focus. CNET.com, the source for computers and technology.

____: This program is funded in part by Northwestern Mutual Life, which has been protecting families and businesses for generations. Have you heard from the quiet company? Northwestern Mutual Life.

____: And by the Corporation for Public Broadcasting and viewers like you.

NARRATOR: Over the last decade, teams of engineers and scientists have been at work in the Italian city of Pisa. A $20 million struggle to save the Leaning Tower is approaching a crucial stage. Built 800 years ago to house the bells of the vast cathedral of the Piazza dei Miracoli, or Place of Miracles, the Leaning Tower of Pisa is one of the wonders of the world. And one of Europe's top tourist attractions, with over six million visitors each year. But the risk of collapse is now so high tourists are banned from entering the Tower. People still travel thousands of miles to steal a glimpse. 180 feet high, with columns, carvings and brickwork all of matching marble, even if it were straight this would be a breathtaking building. There are almost 200 columns supporting the six stories of external arcades. These walkways in the air must have been a thrilling experience for the medieval Pisans. Even today the view is spectacular. The infamous lean is not intentional. The gravity defying five and a half degree tilt is a tragic flaw, that could eventually destroy the Tower. Towers do collapse. In 1989, in Pavia - a few hundred miles north of Pisa, a 14^th century bell tower collapsed, killing four bystanders. Pavia's tower wasn't even leaning. The Italian government reacted, and here in the shadow of Pisa's Tower, a Committee of experts has been assembled. Nobody has refused this invitation. Saving Pisa is the greatest prize in civil engineering. Among the brilliant minds rising to this challenge are Giorgio Macchi of Pavia, and John Burland of Imperial College, London.

JOHN BURLAND: When you come in through the West Gate and see this glistening white structure looming over the Cathedral with its lovely symmetry, it is beautiful, but it's breathtaking, the fact that it hasn't fallen over, that it's just shimmering on the edge.

NARRATOR: Burland, a soil engineer, digs into problems below ground, while Giorgio Macchi, a structural engineer, attacks weaknesses in the Tower itself.

GIORGIO MACCHI: My work is on structural problems so I saw immediately the structural risk. In my opinion the Tower is in real danger.

NARRATOR: Before the Committee intervenes, they must first investigate. So many analytical devices are fixed to the Tower, it becomes the most monitored building in the world. A circular sensor, which can register a vibration from a footstep, runs around the inner walls. Metal clamps detect any shift in 25 of the Tower's largest cracks. Three plumb lines hang from the top level. Taut cross wires measure deformations in the walls. Some startling secrets are exposed - the Tower reacts to changes in the elements. Every day the entire building sways in a minute circle not more than a 100^th of an inch across. This infra-red camera shows why. The lens is sensitive to heat, which is displayed as white. The sun heats the south side more than the north. The marble there expands, creating an imbalance and increasing the lean. At night it will shrink back. Larger movements are recorded during rainstorms.

JOHN BURLAND: We get these extraordinarily heavy storms in Pisa and the water table rises very quickly indeed, and it rises on one side more than on the other and it lifts that side, it happens to be the north side. It lifts the north side a bit, so the Tower goes to the south when the water table comes up.

NARRATOR: When the rain stops, the Tower will move back. Committee member Carlo Viggiani has studied the Tower for 35 years.

CARLO VIGGIANI: At this moment the Tower feels very sad and increases its leaning, but when the sun will come back, she - I say she, probably I must say it, will come back again.

NARRATOR: The sensitivity to the water table suggests there is a problem in the ground below the Tower. This will be an important part of the investigation. But first the Committee faces a more immediate danger. Structural engineer Giorgio Macchi must deal with a series of massive flaws in the Tower's structure. The Tower is a hollow cylinder, 45 feet across. In most places the walls are nine feet thick. But corkscrewing through the walls is a staircase wide enough for two Pisans to climb side by side. This reduces the thickness of the walls to barely a yard in some places. This would be just adequate if these walls were solid marble - as they appear to be. But probes reveal otherwise.

GIORGIO MACCHI: This marble facing is only 25 centimeters thick and behind it there is a conglomerate of stones and lime which has a very, very low resistance.

NARRATOR: Medieval masons often filled in walls with stones and lime but it severely weakens the structure. What's more, there are several holes in the conglomerate, created by the original builder's wooden scaffold.

GIORGIO MACCHI: The holes are some 40 centimeters high, so they reduce the resistance area of the wall in a considerable way.

NARRATOR: It turns out that this elegant marble facing is the Tower's main support. The stress created by the lean falls mainly on this thin façade. That's why many of the bricks are cracked. One may be the start of a collapse. To pinpoint this potential catastrophe, Macchi must locate the Tower's zone of maximum stress. Detailed measurements, and data from the monitors, are fed into a computer which calculates structural stress, and highlights it in blue. The largest stresses are on the south side, below the lean, on the second story, where a doorway meets the staircase. This is where the collapse will begin.

GIORGIO MACCHI: By calculation we found that this is the really critical point.

NARRATOR: If the marble here gives way, the 14,000 ton Tower won't merely crumble. The tensions accumulated over centuries will be unleashed with immense and sudden force. Thankfully this ticking time-bomb has always been well maintained. Even today minor repairs are carried out by Pisan stonemasons who replace some of the damaged marble with newly chiseled blocks.

GIORGIO MACCHI: There were very many substitutions of stones in this area. You can see here is the original stone, and these are completely substituted stones here and there, and completely substituted in this area.

NARRATOR: But traditional maintenance will no longer contain the growing stress of the lean. In 1992 a dozen plastic-coated metal tendons are wrapped tight around the critical second story. The cracks are held shut and the collapse prevented, or at least delayed. This is one of the great monuments of medieval Europe. So the unsightly tendons cannot remain in place here indefinitely. Now that the structure has been strengthened, the Committee begins to search for a permanent solution. The quest carries the engineers into strange territory - deep into the ancient mysteries of the Tower. Medieval Italy was tower mad. Skylines like this one at San Gimignano just south of Pisa were commonplace. For the princes and merchants, the bigger your tower, the more wealth and power you projected. Today Pisa is a quiet university town. But medieval Pisa was an aggressive trading power and one of the richest cities in the world. Piero Pierotti is an historian at Pisa University. Although not on the Committee, he is an expert on the Tower.

[Voice-over translation]

PIERO PIEROTTI: It's said that the most important families in Pisa ate off gold plates, so there's no doubt that some had grown very rich from seagoing trade. And this wealth must have inspired the attempt to do something outstanding for the city.

NARRATOR: In 1064, just inside the north wall of the city, work began on a massive Cathedral. The wedding-cake tiers of marble columns were an early expression of the style that would later embellish the Tower. Inside are more colonnades. Horizontal layers of black and white marble seem to draw worshippers towards the artistic climax - the apse - with its haunting mosaic, Christ In Majesty. The Pisans borrowed ideas from other cultures. There are Byzantine mosaics - and an Islamic dome. And they recycled the marble of classical buildings, inviting comparisons to the glory of ancient Rome. They built what is still one of the largest baptisteries in Italy. Here the richest citizens were christened in style. This bizarre architectural mix has gothic pinnacles sprouting from the trademark Romanesque colonnades. Still, the medieval Pisans weren't satisfied. They decided to give their new Cathedral the most extravagant bell tower the world had ever seen. But their ambition exceeded good building practice.

[Voice-over translation]

PIERO PIEROTTI: While all the other monuments of the Piazza have been signed by their architects, the architect of the Tower is a mystery. Nobody put their name on the Tower. The builders must have felt that the plan for the Tower was over-ambitious, because they knew that the ground in Pisa tended to give way easily.

NARRATOR: This painting reveals the Pisan masons had plenty of experience building Towers. What were the soil conditions that worried them 800 years ago? Between Pisa and the nearby coast is an alluvial marsh. Perhaps this is what the ground around the Tower was like before construction began. Soil engineer John Burland travels to the shoreline to assess what effect this ground might have had upon the Tower.

JOHN BURLAND: Well, about 10,000 years ago the location of the Tower was actually a river estuary. And the tide would come in and out and the floods would come down each year and they'd deposit sand and silt. And the soil beneath the Tower was actually very much like this beach - it's soft - you sink into it. And that's what they actually built the Tower on.

NARRATOR: A team of scientists performs an ingenious experiment to reconstruct Pisa's geological history. A three-foot high aluminum model of the Tower is placed upright on sand which has been carefully arranged to mimic the silt below the Tower. Sandy silt and metal Tower are then lowered into a centrifuge which will simulate the effect of hundreds of years of gravity. Centuries are compressed into a few weeks. When taken out of the centrifuge, this Tower leans just like the real thing. Not only did the model sink, but it sank unevenly - just like the real Tower, which was built on ground no more solid or consistent than a beach.

JOHN BURLAND: And the reason why it's leaning is because the soil on the south side is just a little bit more compressible than on the north side.

NARRATOR: So the Tower settled more deeply into the softer silt on the south side. The scientists have explained the Leaning Tower of Pisa. Now they must save the Tower, by finding a way to halt - even reverse - the lean. Proposals arrive in Pisa from all over the world. Some devised by qualified engineers, others by schoolchildren. But most rely on some version of a huge prop. Aesthetically, this would be unacceptable, and would probably destroy the tower.

JOHN BURLAND: A lot of people say, why can't we just prop it or why can't we just push it back? And this experiment shows why that's about the most dangerous thing you could do, in fact. The masonry is represented by this acrylic cylinder, and this weight here applies weight to the top of the Tower. We'll increase the inclination of the Tower and prop it or push it. And you can see what happens - it just literally collapses.

NARRATOR: The search for a solution takes the engineers back into history. The Tower has always been cared for by an organization called the Opera, whose presidents represent an unbroken line of craftsmen right back to the 11^th century. The Opera archives contain a priceless hoard of information. A report from 1550 showed the top was already 12 feet south of the base. The first scientific measurement was made by two British architects, Edward Cresy and George Taylor. But this was a static observation. Regular surveys have been made since 1911. And these reveal the Tower has stopped settling into the silt. The ground has hardened. The Tower is no longer sinking - it's capsizing.

JOHN BURLAND: And as soon as we discovered that, we thought, ah, if this side is coming up then it's rather like sitting on a sailing boat, if you can just lean out, you can pull it back a little bit. Surely it would be safe to put temporarily some load here to reduce the tendency for the Tower to fall over.

NARRATOR: In July 1993 the engineers begin to act. They load 600 tons of lead ingots on the rising north side. As the lead is piled up, there is always the danger that the ground might give way again. So the Tower's reaction is scrutinized by the electronic monitors.

JOHN BURLAND: They can tell us to one 100^th of a millimeter how much the Tower is moving at the top. So we did this very gradually over a four or five month period. Putting the lead weight on, leaving it for a day, making measurements, putting another one on and so on.

NARRATOR: The lean is brought under control for the first time in 800 years, but the beautiful Tower has become an eyesore. So a permanent solution is devised, with no lead weights. It is based on removing soil from beneath the Tower.

JOHN BURLAND: This model demonstrates the soil extraction technique. We drill in under the Tower with a specially designed drill which causes no disturbance to the ground. And when it's at the location we want it, we can then gently pull the drill out. So if we look at this, the soil will then close in the cavity and we'll see the Tower rotating back towards the north side as we under-excavate. A 10% reduction in inclination will reduce the stresses by 10% in the masonry, and that's a very reasonable amount.

NARRATOR: Reducing the lean by 10% will shift the Tower north by half a degree. In the Piazza, an experiment is done on a concrete test tower which is taller and heavier on the south side and so simulates the forces under the lop-sided Leaning Tower.

JOHN BURLAND: And we were extracting soil over a length of this sort. And you can see here how the ground has subsided, and that brought the whole trial footing northwards by about half a degree.

NARRATOR: But some have doubts about soil extraction.

GIORGIO MACCHI: Such intervention has been tested on a specimen and it works very well. But to extend this intervention under the Tower in this special situation which has been formed in 700 years, there is still a residual risk and the Committee is thinking about that.

NARRATOR: At tense Committee meetings, Carlo Viggiani becomes the first apostle of soil extraction.

[voice-over translation]

CARLO VIGGIANI: If we go back by half a degree, then even if the worst happens - which is that the Tower starts to move again as soon as we leave it - it will take 300 years to get back to the position it's in now.

NARRATOR: But some have no faith in this time machine.

[voice-over translation]

__: I'm a bit puzzled by what Carlo's saying. Are you suggesting we can go back in time 300 years?

NARRATOR: There is so much at stake, it's hard to reach an agreement - especially with so many experts.

CARLO VIGGIANI: What is very peculiar about this Committee is that it is a multi-disciplinary committee. There are people from the side of history of art, of restoration and engineers, geo-technical engineers, structural engineers, and it is not easy to work together for such differently minded people.

NARRATOR: Before approving soil extraction, the fractious Committee demands conclusive evidence that this intervention will not damage the Tower. The soil engineers must establish precisely how this fragile structure reacts to changes in the ground below it. Yet again, the answer lies in the past. Jane Morley is an architectural historian who has studied the Tower extensively.

JANE MORLEY: I don't think anyone short of God will understand what it took to build this Tower, and why it's doing what it's doing now. The people that built this were not scientists or engineers. They probably had very little formal education - they were builders. They had a lot of knowledge, knowledge that isn't quantified or codified. It's what you learn by doing, it's what you learn by being a builder.

NARRATOR: Perhaps something the medieval builders did will give modern engineers useful insights into the mysterious interaction between this structure and the soil. The first clue is in the impressive craftsmanship of the exterior marble.

CARLO VIGGIANI: Each stone layer is precise within less than a millimeter, and this is the reason why we can get so many information just by measuring these stones. You can see that, in most of the stones there is no mortar at all at the contact between two stones. It depends on the high quality of workmanship.

NARRATOR: These stones reveal an extraordinary history. Construction of the Tower began in 1172. After about six years the masons had reached the fourth level, but then they stopped. The Tower was abandoned for almost a century. Probably this was due to political and economic strife. But perhaps the delay was really a brilliant strategy.

JOHN BURLAND: If they'd built the Tower in one go, it literally would have fallen over as this one is. You can just see it settling down. It's just collapsed. And that's what would have happened if they'd built it all in one shot. So what they actually did was, they built the Tower up to a little above the third story and then they stopped. And the weight of that Tower in that state squeezed the underlying ground, and over the years the ground became stronger. So when they came back 100 years later it could take the full weight of the Tower.

NARRATOR: When construction did resume, the masons must have noticed a slight lean to the north. So they tried to straighten the Tower.

CARLO VIGGIANI: What they did is to have slightly higher stones on the side where the Tower was leaning.

NARRATOR: The corrections are so small they cannot be seen with the naked eye and were only discovered recently by detailed surveys. But as the masons straightened the Tower, it lurched in the opposite direction. When it reached the seventh story, work stopped again. Over the next 90 years the southward lean grew to over one and half degrees, a severe inclination that did not deter the masons from adding a heavy bell chamber.

CARLO VIGGIANI: When they started again to construct the bell tower, they had to make a substantial correction and they did this by changing the number of steps according to the direction. In this side - we are on the north side - we have only four steps. We will see that on the opposite side we have six steps. Now we are on the south side where the steps are six, just to correct for the inclination. I feel this Tower personally as a challenge. I cannot conceive the men constructing it knowing that it was obviously leaning and going on and finishing it.

NARRATOR: As a result of the delays and corrections over generations, the Tower appears banana-shaped. This deformity provides the vital clue to the relationship between the soil and the structure - and is the basis of Burland's computer model.

JOHN BURLAND: We've been through a period of about three years of very, very intense study using our computer models studying soil extraction. All the results have been positive. We've learnt what we can do and what we can't do.

NARRATOR: But some believe the computer model oversimplifies the construction of the Tower.

JANE MORLEY: You had different people working on it at different times with different states of knowledge about what structures like this can do, what they can't do, what happens to them over time. And you've got different sections of the Tower that are different geometries. So to right it any degree, you may not get the predicted structural action that you would want.

NARRATOR: The proof the Committee demands remains elusive. In the meantime, its deliberations are complicated by yet another proposal, a plan to replace the unsightly tendons. In his lab, structural engineer Giorgio Macchi experiments on a three-quarter inch thick steel bolt that fits inside the marble blocks - stiffening them from within. But there is a risk the bolt might actually weaken the marble. On a test rig, a marble block is forced upwards, while the bolt is held in a vice. Eventually one will break. But Macchi has calculated it will not be the marble. The force builds up to 12 tons. Then the bolt snaps, leaving the marble unharmed, suggesting a way to strengthen the Tower without defacing it.

GIORGIO MACCHI: We should do something very quickly. We know which are the problems of the Tower and I think that further research could not improve very much our knowledge now.

NARRATOR: So, there are two long term solutions for the Tower. One would stabilize the structure with internal bolts, the other would reverse the lean through soil extraction. Faced with this choice, the Committee hesitates, their indecision fueled by a frightening history. There have been many previous interventions. All have failed. The disastrous meddling began with the digging of this trench around the base of the Tower.

JOHN BURLAND: This is called the Catino. It's a walkway, and it was excavated by an architect called Gherardesca in 1838. Now it's important to remember that the Tower has settled about three meters since it was built.

NARRATOR: As a result, the elegant carvings at the base of the columns on the ground floor had sunk below ground.

JOHN BURLAND: And Gherardesca argued that it would be lovely to reveal them so that people walking around the Piazza could view the base of the columns as the original architect had intended them to be. So he just came in and dug this out. And he dug down about one and a half, two meters down. And this is the wall of the Catino. What he didn't realize was that the water table is about here. So there's a lot of water behind this wall and under the foundations. And as he dug this Catino out, the water came spouting out of the ground. And the Tower moved, the top of the Tower moved about half a meter. It lurched literally this way. It's amazing that it didn't fall over.

NARRATOR: There was more agony in the 1930s. Benito Mussolini, leader of the Fascists, had become dictator of Italy. His regime rejected the drooping Tower as an inappropriate symbol. Engineers were ordered to sort out the troublesome lean. A plumb line was installed to measure it in thousandths of a degree. But below ground the engineers wreaked havoc. They drilled holes through the floor of the Tower, and almost 200 tons of concrete were poured into the foundations. The new plumb line recorded a southward lurch of nearly a tenth of a degree. The work had destabilized the delicate Tower. In the 1950s, the seven swaying medieval bells were locked tight. Their vibrations had been shaking the Tower apart. So precarious was the condition of the Tower that even its bells could tip the balance. An object lesson that even today must be taken to heart.

JOHN BURLAND: When anybody has tried to do anything on the south side, the Tower has always said, don't touch me, I'm very, very delicate, and it's moved. So the Tower has actually spoken to us through the way that it's moved.

NARRATOR: Caution is wise, but visitors grow impatient. Pisa's tourist industry appears to be struggling. Eventually, the continuing threat of an earthquake and the hideous lead ingots trigger a major political shift and the green light is given to a high risk proposal.

GIORGIO MACCHI: The Committee would like first to remove the lead weights and to substitute them with invisible cables in the soil, giving the same effect.

NARRATOR: In this plan, anchors will be attached to a concrete ring wrapped around the base of the Tower. The engineers will drill through the soft soil and secure the anchors in the solid bedrock 130 feet down. The pull from the anchors will replace the push of the unattractive lead weights. This operation will not reverse the lean, but should halt and stabilize it. The contractors first attempt to install the ring through which the anchors will be connected to the Tower. Made of reinforced concrete, the ring is positioned under the floor of the Catino - well beneath the water table. This is a major gamble.

GIORGIO MACCHI: Now we are very nervous. We have always been very nervous when we did any intervention on this Tower because the Tower is reacting in a way which is not always the way you expect.

NARRATOR: As the contractors burrow down, they inject liquid nitrogen, at a temperature of about 200 degrees below zero, into the ground. This freezes the surrounding ground water so that it cannot flood the excavation. During the freezing, Opera president Ranieri Favilli, all too familiar with the consequences of past interventions, gets carried away by an overactive imagination.

[voice-over translation]

RANIERI FAVILLI: There was a period in which, to tell the truth, I imagined I heard some noises that were a little strange during the night. I can't say I didn't worry about this. Unfortunately it's inevitable, because first of all I'm a Pisan. Secondly I would frankly be very unhappy to go down in history as the Opera president under whom the Tower fell.

NARRATOR: As the freezing moves to the sensitive south side, the monitors watch the Tower closely. At 3:30 a.m. on the 6th of September 1995, the Tower reacts. The top lurches a 16^th of an inch south.

JOHN BURLAND: In one night it moved what it normally moved in a year. And that worried us because if that had continued that would have been very large, and the Tower's stability may have been in jeopardy.

GIORGIO MACCHI: We knew that the Tower was moving, was responding in a special way and we decided to stop the work.

NARRATOR: Contractors scramble to restabilize the Tower by loading on more lead ingots. The lurch is halted, but the Committee's attempt to remove the ugly lead has resulted in an even larger pile around the Tower's base.

[voice-over translation]

PIERO PIEROTTI: This Committee, what has it done? Intervened, basically, in that same highly delicate zone where Gherardesca had intervened. And all they did is bring the Tower closer to collapse. So there you are. They've been working for several years, which is too long, and now they've left the Tower in a worse condition than they found it in.

JANE MORLEY: One would believe if one were of a late 20^th century technocratic mentality or just simply have a faith in science, well, we know so much, why can't we come up with a solution to the problem? But there may be in fact no solution.

NARRATOR: The building site around the Tower lies quiet for a year after what becomes known as Black September. Then in late 1996, because of a dispute within the Italian government, the Committee is disbanded. Burland's extensive research now looks like it has been a wasted effort.

JOHN BURLAND: It's been very frustrating. A lot of my friends say to me, how can you possibly stand the way this has gone on and on and on, why don't you just resign? It would be so much easier. But it's so important to the life of the Tower that we do something soon.

NARRATOR: The Committee remains out of action until the fall of '97. Just north of Pisa, the area around Assisi - birthplace of St. Francis - is hit by an earthquake measuring almost six on the Richter scale. Thousands are left homeless. Unique historic buildings are damaged beyond repair. And it soon emerges that although this earthquake was no surprise, little was done to prepare for it. The resulting scandal inspires a new dynamic attitude in Italian restoration and conservation. In Pisa, the Committee reconvenes, and approves soil extraction.

JOHN BURLAND: Now we're faced with the reality of doing something on the Tower for the first time. Something very delicate, and something that we hope will provide the final solution.

NARRATOR: But there is no margin for error. So the engineers on the Committee devise one of the strangest contraptions in this bizarre story. 100 yards north of the Tower, just behind the Opera, contractors begin work. This is one of two tripods designed to hold steel cables that will run right across the Piazza to brackets on the Tower's second story. The cables will form a massive harness to support the Tower.

JOHN BURLAND: A harness is a temporary safeguard structure. It's there to make quite certain that if something goes wrong we can control it. It's only temporary. It'll only be there when we're operating on the Tower.

NARRATOR: During an unsettling deluge, the first cable of the harness is craned into position. Although only temporary, this is a huge project. Just ten yards of this cable weighs half a ton. If it lashes out from the crane, it could kill the workmen and damage the fragile Tower. At a critical moment, the cable slips and jams into a marble arch. Locals come out to watch and hope that the Tower will not overreact. Hours later, the struggle to free the cable is successful. That night the first component of the harness is secured. The completed harness is carefully balanced and adjusted. It looks like heavy engineering, but there's only enough force exerted on the Tower to resist a minor mishap.

JOHN BURLAND: The harness is not intended to stop a catastrophic failure, it's not for that at all. It's simply to hold the Tower gently if the movements of the Tower are unexpected.

NARRATOR: The scene is set and the soil extraction is put to the test.

JOHN BURLAND: We have just started soil extraction. The drill is about five meters below ground level. And then the soil extraction takes place beneath the floor of the Catino. And the effect extends out, as we take the soil out, the effect of it extends under the Tower.

NARRATOR: The success of soil extraction hinges on the assumption that the ground below the Tower is mainly silt. On day one, soil begins to spew out of the extractor casing. But it looks like clay, not silt. Burland is eager to make an on-the-spot analysis.

JOHN BURLAND: An experienced soil engineer can tell exactly what the soil material is. If you just put a grain or two against your teeth, you can tell straight away that this is actually a sandy silt. It feels like clay but it's actually a sandy silt. So it's total confirmation of what we were expecting.

NARRATOR: The Tower has been creeping south for centuries, so the lean will not be reversed in a few hours. Right now, the main fear is that the drilling will trigger a catastrophic lurch further south. But the Tower is quiet, and the operation continues.

JOHN BURLAND: We have started what could well turn out to be the final stabilization measures. The result will be that the Tower will be leaning at 10% less than it is now and we will have added 300 years to its life at least, and we will have reduced the stresses in the masonry. And these have been our prime objectives right from the start of the whole project.

NARRATOR: After one month of soil extraction the top of the Tower has moved a fifth of an inch north. But the stable position of 300 years ago lies another foot away. Even if the operation goes well, it will take about two years to coax the Tower that far north. Until then, the Leaning Tower of Pisa will remain wired-in and strapped-up like a patient on the operating table. Its progress will be anxiously watched, lest this enigmatic structure confronts us with an unwelcome surprise.

____: The Leaning Tower. The Statue of Liberty. Windsor Castle. And the Parthenon. On NOVA's Website, find out what it takes to rescue the world's most famous monuments.

____: Next week on NOVA: can we take a shortcut through outer space? Or a thrill ride back in time? It's more than just fantasy, it's real science. The secrets of time travel.

____: To order this show or any other NOVA program for $19.95 plus shipping and handling, call WGBH Boston Video at 1-800-255-9424. NOVA is a production of WGBH Boston. Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.

____: This program is funded in part by Northwestern Mutual Life, which has been protecting families and businesses for generations. Have you heard from the quiet company? Northwestern Mutual Life.

____: CNET, bringing the digital age into focus. CNET.com, the source for computers and technology.

____: And by the Corporation for Public Broadcasting and viewers like you.

____: This is PBS.