The cathedral in Pisa has a free-standing bell tower on which construction began August 9, 1173 and took 117 years to complete. Built on a small foundation in soft soil, however, the massive tower soon began to tilt and over the centuries came to be know as the Leaning Tower of Pisa. Through the centuries many plans were devised to correct the tilt, but none were ever determined to be feasible given the size of the massive structure. In 1987, the Tower became a UNESCO World Heritage site. By 1990, however, engineers determined that the increasing tilt was a danger to visitors and on January 7, 1990 the Leaning Tower of Pisa was closed to the public. Engineers removed the bells from the tower to decrease the weight and stress on the building. By removing soil from the raised side of the Tower, a correction of 18 inches was achieved. The tower was re-opened to the public on December 15, 2001.