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Tower of Pisa (Torre di Pisa) or simply the Leaning Tower of Pisa (Torre pendente di Pisa) is the campanile, or freestanding bell tower of the cathedral of the Italian city of Pisa, known worldwide for its nearly four-degree lean, the result of an unstable foundation. The tower is situated behind the Pisa Cathedral and is the third-oldest structure in the city's Cathedral Square (Piazza del Duomo), after the cathedral and the Pisa Baptistry. The height of the tower is 183.27 feet from the ground on the low side and 185.93 feet on the high side. The width of the walls at the base is 8 feet. The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase. The tower began to lean during construction in the 12th century, due to soft ground which could not properly support the structure's weight, and it worsened through the completion of construction in the 14th century. By 1990 the tilt had reached 5.5 degrees. The structure was stabilized by remedial work between 1993 and 2001, which reduced the tilt to 3.97 degrees. Between 1589 and 1592, Galileo Galilei, who lived in Pisa at the time, is said to have dropped two cannonballs of different masses from the tower to demonstrate that their speed of descent was independent of their mass. The primary source for this is the biography Racconto istorico della vita di Galileo Galilei (Historical Account of the Life of Galileo Galilei), written by Galileo's pupil and secretary Vincenzo Viviani in 1654, but only published in 1717, long after his death. During World War II, the Allies suspected that the Germans were using the tower as an observation post. A U.S. Army sergeant sent to confirm the presence of German troops in the tower was impressed by the beauty of the cathedral and its campanile, and thus refrained from ordering an artillery strike, sparing it from destruction; in 1987, the whole square was declared a UNESCO World Heritage Site / Pisa is a city and comune in Tuscany, central Italy, straddling the Arno River just before it empties into the Ligurian Sea. It is the capital city of the Province of Pisa. Although Pisa is known worldwide for its leaning tower (the bell tower of the city's cathedral), the city of over 100,000 residents contains more than 20 other historic churches, several medieval palaces, and various bridges across the Arno. Much of the city's architecture was financed from its history as one of the Italian maritime republics. The city is also home of the University of Pisa, which has a history going back to the 12th century and also has the Scuola Normale Superiore di Pisa, founded by Napoleon in 1810, and its offshoot, the Sant'Anna School of Advanced Studies, as the best-sanctioned Superior Graduate Schools in Italy