Construction began on the bell tower of Pisa's cathedral in the 12th century, but was halted when it was only three stories high because of war between Pisa and other Italian cities.
Even at three stories, the structure began to sink because of the unstable subsoil upon which it was built. "Imagine a tower founded on ground which has the consistency of jelly or foam rubber to great depth," says J.B. Burland, a British professor of soil mechanics.
During construction of the upper floors of the tower, which began in 1272, engineers tried to design the upper floors so that their weight would correct the lean. Instead, their efforts caused the tower to lean in a different direction -- south instead of north.
After additional stops and starts, the tower was completed in the 14th century, including a bell tower on top. During the centuries afterward, the structure continued to lean further south; by the mid-20th century, there was considerable concern that it would fall.
After lengthy consideration about what to do to save it, during the late 1990s engineers hired by the Italian government removed tons of soil from underneath the north side of the tower, straightening it a little, though not visibly.
As of the early 21st century, engineers say that the tower is no longer moving and will retain its current lean for at least another two centuries.
