The Leaning Tower of Pisa began tilting while it was still being built, and took 199 years to finish.

The tower stands on soft subsoil of clay, sand and silt, with a foundation only about three meters deep. As soon as the weight of the third floor was added, the ground on the south side began to compress more than the north, and the tower tipped. Then war broke out between Pisa and rival cities, and construction stopped for almost a hundred years.

That pause probably saved the tower. It gave the soil time to settle and become firmer under the load. Had building continued straight away, many engineers think the tower would have collapsed. When work resumed, builders tried to correct the lean by making the upper floors slightly taller on one side, which gave the tower a gentle curve.

The bell chamber at the top was added in the 1300s, completing the tower about 199 years after work began.

The tower is the freestanding bell tower, or campanile, of Pisa Cathedral, part of the Piazza dei Miracoli complex. It rises about 56 meters and holds seven bells. Over centuries the lean grew steadily, and by 1990 it had reached about 5.5 degrees, so the top was offset several meters. Engineers feared collapse, and the tower was closed to the public.

An international committee led a long stabilisation project. The most effective step was soil extraction: carefully removing small amounts of soil from beneath the north side, so the tower would settle back slightly. By the time it reopened in 2001, the lean had been reduced by about 45 centimeters at the top, to roughly 4 degrees. Monitoring since then shows it is stable.

Today sensors monitor the tower constantly, tracking tilt, temperature and ground movement. Engineers report that it has even straightened very slightly since the stabilisation work ended.

Experts estimate the tower should remain stable for at least two centuries without further major intervention, provided the ground conditions do not change.

Pisa's tower is not alone. The same soft ground affects other buildings in the square, including the cathedral and baptistery, which lean slightly too. Elsewhere in Italy and Europe, many medieval towers and churches tilt for similar reasons, though few as dramatically.

What makes Pisa's case special is that people embraced the flaw. The lean turned an ordinary bell tower into a world famous landmark, which is why engineers aimed to stabilise it, not straighten it.

The tower has become one of the most photographed buildings on Earth, largely thanks to visitors posing as if they are holding it up. Its fame has made Pisa a major tourist destination, which is exactly why preserving the lean mattered.