Legend holds that Galileo conducted an experiment on the nature of gravity by dropping metal balls from the top of the 187-foot-high Leaning Tower of Pisa. Historians, however, say this legend has no basis in fact—which isn't quite to say that it's false. Work on this tower, built as a campanile (bell tower) for the Duomo, started in 1173: the lopsided settling began when construction reached the third story. The tower's architects attempted to compensate through such methods as making the remaining floors slightly taller on the leaning side, but the extra weight only made the problem worse. The settling continued, and by the late 20th century it had accelerated to such a point that many feared the tower would simply topple over, despite all efforts to prop it up. The structure has since been firmly anchored to the earth. The final phase to restore the tower to its original tilt of 300 years ago was launched in early 2000 and finished two years later. The last phase removed some 100 tons of earth from beneath the foundation. Reservations, which are essential, can be made online or by calling the Museo dell'Opera del Duomo; it's also possible to arrive at the ticket office and book for the same day. Note that children under eight years of age are not allowed to climb.