Bonnano Pisano, the engineer in charge, sought to compensate for the lean by making the new storeys slightly taller on the short side. But the extra masonry caused the structure to sink even further. Work was suspended several times as engineers sought solutions, but the tower was ultimately topped in the 14th century, still leaning.

What is a barrier reef?

It is a line of corals roughly parallel to a shore and separated from it by a lagoon or other body of water. A barrier reef is usually pierced by several channels that give access to the lagoon and the island or the land beyond it.

One of the most famous is the Great Barrier Reef off the northeastern coast of Australia. It extends in a northwest-southeast direction for more than 2,000 kilometres at an offshore distance ranging from 16 to 160 kilometres. It has an area of some 350,000 square kilometres.

The reef actually consists of some 2,100 individual reefs and some 800 fringing reefs. Many are dry or barely awash at low tide.

The reefs have been formed over millions of years, from the skeletons of a mass of living marine organisms.

European exploration of the reef began in 1770, when British explorer Captain James Cook ran his ship aground on it. A modern laboratory on Heron Island continues scientific studies.

In addition to its scientific interest, the reef has become increasingly important as a tourist attraction. The extensive use of tourist craft and activities such as drilling for pe troleum have caused growing concern over the preservation of its natural heritage. In 1981, the Great Barrier Reef was added to Unesco's World Heritage List.

Graphic: SPBRAIGLO