It has been around for hundreds of years, and is world famous because it moved. It is still moving at the rate of 1-2 millimeters per year, and this has people worried. What is it?

This iconic marble bell tower, or campanile, in Pisa, Italy, earned its fame through a near-disastrous flaw. Construction began in 1173 on a shallow foundation (Review) set in soft, unstable subsoil. The structure began to sink and tilt almost immediately, after work had progressed to only the second floor. The lean was so obvious that construction was halted for nearly a century, but the problem was never truly solved, creating the accidental architectural wonder we know today.

Over its nearly 200-year construction period, engineers attempted to compensate by making new floors taller on one side, but this only worsened the tilt by adding more weight. By the late 20th century, the lean had become so severe that there were legitimate fears the tower would collapse. This worry prompted a massive stabilization project between 1990 and 2001, where soil was carefully removed from underneath the high side. The effort was a success, slightly correcting the lean and securing the foundation. While it still shifts minutely, the tower is now considered stable for at least the next 200 years.