The leaning tower of Pisa is a UNESCO World Heritage Site that attracts millions of tourists each year. Even though it became famous because it leaned, this also was a cause for concern. The tilt began during construction because of an inadequate foundation on ground too soft on one side to support the weight. The building was built with a deliberate tilt from the fourth floor on. This was made worse by measures to prevent the tower from collapsing. The tower was finally stabilized and made safe for tourists in the 1990s.