Leaning Tower of Pisa

The most famous accident in architecture. Begun in 1173 as the bell tower of Pisa's cathedral, it started sinking into soft ground before the third storey was up, and builders spent two centuries trying to correct it, curving the tower slightly as they went. Galileo is said to have dropped weights from the top to test gravity. By the 20th century it leaned more than five metres and was nearing collapse; engineers quietly removed soil from the high side in the 1990s to ease it back. It still leans, on purpose now, just enough.

Description via source. Coordinates and heritage data from the Atika atlas.