The Leaning Tower of Pisa

Workers laid the foundation stones on 9 August 1173, in a circular ditch dug into ground made of clay, fine sand and shells.

Sources differ on how deep the foundation goes, from about five feet (1.5 metres) to 3 metres to about 10 feet.

The tilt was clear by 1178, when the tower was just over three storeys tall and building stopped.

Modern analysis shows that if builders had carried on before the underlying soil consolidated, the tower would certainly have toppled.

Pisa was often at war with Florence, Genoa and other city states, so work was slow and sometimes abandoned for decades.

Work restarted in 1272 under Giovanni di Simone, nearly a century after the tower was begun.

By 1278, when workers reached the seventh cornice, the tower leaned to the south by about one degree, roughly 2.7 feet.

Builders tried to correct the lean as they went. The bell chamber's floor got six steps on the south side and only four on the north, and upper floors were built with one side shorter than the other, so the tower is curved as well as tilted.

The tower was finished in 1372, after nearly 200 years of work, and it is 56 metres tall.

The completed tower weighs more than 14,000 tons.

In 1817 two British architects used a plumb line and found the lean was about five degrees.

Annual measurements of the lean with a theodolite began in 1911.

In 1934 workers injected grout into the foundation masonry to strengthen it, and the tilt suddenly got worse.

By 1989 the lean had reached 5.5 degrees. That year a similar tower in Pavia collapsed, which spurred serious action.

The tower closed to the public in 1990. The Italian government appointed a committee of 13 experts, led by the British engineer John Burland.

Soil extraction began in February 1999. By the end of August the lean had decreased by 130 arc seconds (1.5 inches).

Engineers used a special drill 200 millimetres in diameter, sunk at an incline beneath the north side of the foundation, to remove small amounts of soil in a controlled way. The tower leans to the south.

The work, completed in 2001, straightened the tower by 38 centimetres and left it leaning about 3.97 degrees. The tower reopened to the public in 2001.

A 2013 study found the tower stable but tending to straighten, and it had moved a further 2.5 centimetres vertically since 2001. Engineers believe its future is secure for at least 200 years.

The tower stands in the Piazza del Duomo with the cathedral, baptistery and cemetery. UNESCO inscribed the site in 1987.