Why is the Leaning Tower of Pisa leaning? The soft clay, the 199-year build, the 5.5-degree tilt and how engineers stopped it falling

Architecture · 4 September 2026

The Leaning Tower of Pisa leans because it was built on about three metres of foundation in ground that is soft clay, sand and mud, the former bed of the River Arno, and the ground under its south side compressed more than under its north: the bell tower of Pisa Cathedral, begun in 1173, started to tilt as soon as the third storey was finished in 1178, and every later builder tried to correct it by building the next storeys with one side taller than the other, which is why the tower is slightly curved. It took 199 years to complete, with two pauses of decades caused by wars, which let the clay settle and are probably why it did not fall. By 1990 it leaned 5.5 degrees, 4.5 m out of vertical at the top, and was closed as near collapse; between 1990 and 2001 engineers removed 38 cubic metres of soil from under the north side and pulled it back to about 4 degrees, where it has been stable since. Below: the ground, the building, the tilt, the fixes, the numbers, and the stories, with the cathedral it belongs to in the Buildings by style collection.

The ground

Pisa stands on the flood plain between the Arno and the Serchio, on ground the rivers laid down: under the Piazza dei Miracoli the first 10 m are silty sand and clay, below that 30 m of soft marine clay, the "Pancone", and only below 40 m dense sand. The whole square is soft, and the cathedral and baptistery also tilt, the baptistery by 0.6 degrees. The tower's builders dug a ring foundation about 3 m deep and 19.6 m across for a tower that would weigh 14,500 tons, which put a pressure of about 500 kilopascals on clay that could bear less, and the clay under the south side happened to be weaker and wetter than under the north. The lean is therefore not a fault in the tower but a fault in the ground, and it began as soon as the weight was applied; had the tower been built in a few years to its full height, the calculations of the 1990s commission suggest, it would have fallen during construction. Leaning Tower of Pisa.

The building, 1173–1372

• 1173. Work begins on 9 August, on the free-standing bell tower of the Romanesque cathedral, the fourth building of the cathedral square, a cylinder of white Carrara marble with arcaded galleries on every storey; the architect is unknown, and the traditional attribution to Bonanno Pisano rests on a name found in a piece of stone near the base in 1820.
• 1178. At the third storey the tower begins to tilt, north at first, then south, and work stops; Pisa is at war with Genoa, Lucca and Florence for the next century, and the tower stands unfinished for 94 years while the clay under it consolidates.
• 1272. Giovanni di Simone resumes work and builds four more storeys, with the south-side columns and arches taller than the north to bring the tower back toward vertical; the tower now curves like a banana, and the tilt continues.
• 1284. Work stops again after Pisa's fleet is destroyed by Genoa at the Battle of Meloria; the tower stands at seven storeys for nearly a century.
• 1319–1372. The bell chamber, the eighth storey, added by Tommaso Pisano, again offset to the north; the tower is complete at 56 m, with seven bells, the largest of 3.6 tons, cast in 1655; the tilt is about 1.6 degrees.

The style, and the cathedral group it belongs to with its cathedral, baptistery and walled cemetery, is described in Romanesque architecture; the cathedral, begun in 1063 with the spoils of a victory over the Saracens, is in the collection.

The tilt, 1372–1990

The tower went on leaning at about 1.2 mm a year, a millimetre or so more each century, as the clay crept under the load. Measurements begin in 1817, when two English architects recorded 5 degrees; in 1838 Alessandro della Gherardesca dug out the base of the tower, which had sunk into the ground, to expose the foundation and build a walkway, hit water, and caused a sudden extra tilt of several centimetres. In 1934 Mussolini ordered the tower straightened and engineers drilled 361 holes in the foundation and injected 90 tons of cement, which made it lean more; in 1966 and 1985 further drilling caused further lurches. By 1990 the top was 4.5 m out of vertical, the tilt was 5.5 degrees and accelerating, and the masonry on the south side was under a stress close to the crushing point of the marble; the collapse of the Civic Tower of Pavia in 1989, a similar medieval tower that fell without warning and killed four people, decided the government, which closed the tower on 7 January 1990 and appointed a commission of engineers under Michele Jamiolkowski, with John Burland of Imperial College as the soil expert.

How it was saved, 1990–2001

• 1992. Steel cables were wrapped around the second storey to hold the masonry, and in 1993 600 tons of lead ingots were stacked on the north side of the base as a counterweight; the tower moved back by 1 cm. A 1995 attempt to replace the lead with anchors in the deep sand, using liquid nitrogen to freeze the ground, made the tower lurch south by 1.5 mm in one night and was abandoned; more lead was added.
• 1998. Two steel cables were fixed from the third storey to anchors 100 m away to catch the tower if it moved during the next step.
• 1999–2001. Soil extraction: 41 drills were driven under the north side at an angle and small quantities of clay removed, a few litres at a time, so that the north side settled and the tower rotated back toward vertical under its own weight. Over two years 38 cubic metres of soil were taken out and the tower came back 45 cm at the top, from 5.5 to about 4 degrees, the tilt of 1838; the lead and the cables were removed and the tower reopened on 15 December 2001.
• 2001–2008. A drainage system was installed to keep the water table under the north side steady, and monitoring showed the tower stopped moving in 2008 for the first time in its history. It has since straightened by a further 4 cm on its own.

The commission's aim was never to make the tower straight, which would have removed the reason for coming to Pisa, but to take it back to a tilt it had survived for centuries. Burland estimated it is now safe for at least 200 years.

The numbers

• Height. 55.86 m on the low side, 56.67 m on the high side; eight storeys, 294 steps on the north side and 296 on the south; the walls 2.4 m thick at the base.
• Weight. 14,500 tons on a foundation 19.6 m across and 3 m deep.
• Tilt. About 3.97 degrees today, 3.9 m out of vertical at the top; 5.5 degrees in 1990.
• Time. 199 years, 1173–1372, of which about 25 were building.
• Fix. 11 years and about 30 million euros, 38 cubic metres of soil, 45 cm back.
• Visitors. About 5 million a year to the square, with entry to the tower limited to 45 people every 15 minutes on a timed ticket; children under eight are not admitted, and the climb takes about half an hour.

Versions and theories

• Galileo dropped balls from it. The story that Galileo, a lecturer at Pisa in 1589–92, dropped two balls of different weights from the tower to show they fall together comes from a biography written by his pupil Viviani after his death; Galileo never mentions it, and historians consider it a thought experiment or a legend. A Dutch experiment of 1586 from a church tower in Delft did the same thing first.
• It was built leaning on purpose. No: it was built on soft ground, tilted, and was corrected by its later builders; the curve in it is the evidence.
• The wars saved it. Yes, in the sense the engineers accept: the two long pauses let the clay consolidate under the partly built tower, and a tower built continuously would have failed.
• It will fall in our lifetime. Not on the evidence: it has been stable since 2008, and the tilt is less than it was in 1838.
• Mussolini made it worse. True: the 1934 cement injection is on record as increasing the tilt, as were the excavations of 1838 and the drilling of 1966 and 1985. Every intervention before 1990 harmed the tower.
• Other leaning towers. Pisa's is not the most tilted: the church tower of Suurhusen in Germany leans 5.19 degrees, and the bell tower of Bad Frankenhausen 4.9; the Garisenda in Bologna, 4 degrees, was closed in 2023 as dangerous. Pisa is the most famous because it is the most beautiful.

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