The Leaning Tower of Pisa started tilting while it was still being built. Workers noticed the problem in 1178, when the tower was only two storeys tall.
The tower is the bell tower of Pisa Cathedral in Tuscany, Italy. It took almost two hundred years to finish, and the lean is the reason why. Every time builders returned to the job, they had to work around a tower that was already going the wrong way.
Why does the Leaning Tower of Pisa lean?
The tower leans because its foundation is too shallow for the ground underneath it. The foundation is only about three meters deep, and it sits on weak, unstable subsoil (Institution of Civil Engineers).
Here is what that means in practice. Soft ground compresses under weight. It did not compress evenly under the tower, so one side sank further than the other. The tower did not fall over, but it tipped.
- Construction began in August 1173.
- The tower began sinking in 1178, at the second floor.
- Builders stopped, and the site sat unfinished for roughly 55 years.
- Work resumed in 1233, stopped again in 1278, and resumed in 1319.
- The tower was completed in 1372.
The long pauses may have saved it. While work was halted, the soil underneath had time to settle and strengthen. A tower built straight through in ten years would likely have toppled.
How tall is the tower, and how far does it lean?
The tower measures 55.86 meters on its low side and 56.67 meters on its high side. The difference between those two numbers is the lean.
It currently tilts at about 3.97 degrees. Inside, a spiral staircase of 296 steps climbs to a belfry holding seven bells.
One boundary worth naming: the tower is not the tallest or the oldest bell tower in Italy, and it is not the only leaning tower. Pisa has two other tilting towers. This one is famous because the tilt is extreme and the building survived it.
What did engineers do to stop it falling?
Engineers removed soil from beneath the raised side of the tower so that side could settle down to meet the sunken side. The method is called soil extraction, and it is the opposite of what most people expect.
The tower closed to visitors on 7 January 1990. Over the next eleven years, engineers drilled inclined holes under the north side of the foundation and removed small amounts of soil in controlled stages. About 70 metric tons of soil came out. They also installed drainage to stop winter groundwater lifting the north side further.
The Institution of Civil Engineers reports the tower straightened by 38 centimeters, and that engineers believe its future is secure for at least 200 years. The tower reopened on 15 December 2001.
A correction worth making
The claim: Many sources say the Leaning Tower of Pisa is still tipping further every year and will eventually fall.
Why it persists: The tilt did increase for centuries, and news coverage before 1990 described the tower as close to collapse. That framing stuck.
The correction: The stabilisation project reversed part of the lean rather than merely slowing it, and the engineers who did the work expect the structure to stay stable for at least two centuries (Institution of Civil Engineers).
What to do with that: Teach the tower as a solved engineering problem, not a disaster waiting to happen. The interesting question is not whether it will fall. It is how you move a stone tower by taking dirt out from under it.
Using the tower in a lesson
The tower works well as a bridge between history and physical science. Three angles that hold a class:
- Center of gravity. A tower stays up while its center of gravity falls inside its base. Students can test this with stacked blocks on a soft surface.
- Soil as a material. Ground is not the same everywhere. Comparing how weights settle into sand, clay and gravel makes the Pisa problem concrete.
- Time and consequence. A decision made in 1173 was still being managed in 2001. Few buildings show cause and effect across eight centuries so plainly.
The tower sits inside a wider story about building and power in medieval Europe, when cities competed to raise the tallest cathedral and bell tower. For more on the period and the people who traveled out of it, see our world history section and the pages on explorers.
Quick facts
| Location | Pisa, Tuscany, Italy |
|---|---|
| What it is | Bell tower of Pisa Cathedral |
| Construction began | August 1173 |
| Completed | 1372 |
| Lean first noticed | 1178, at the second floor |
| Height | 55.86 m low side, 56.67 m high side |
| Steps | 296 |
| Bells | 7 |
| Current tilt | About 3.97 degrees |
| Closed for stabilisation | 7 January 1990 to 15 December 2001 |
Sources
- Institution of Civil Engineers. Why Is The Tower Of Pisa Leaning?
- Wikipedia. Leaning Tower of Pisa, construction chronology and dimensions.