Tourists have craned their necks—and scratched their heads—for centuries to uncover why the Leaning Tower of Pisa leans. As it turns out, that famous tilt is the result of a bad foundation. The tower was built in the early 12th century on ground too soft on one side to support its weight. As each successive story was added, the foundation destabilized—eventually displacing the tower by five and a half degrees. Standing today at a corrected four-degree tilt, the lopsided landmark is lauded by architecture buffs as one of history's most beautiful mistakes.