<P> A multinational task force of engineers, mathematicians, and historians gathered on the Azores islands to discuss stabilisation methods . It was found that the tilt was increasing in combination with the softer foundations on the lower side . Many methods were proposed to stabilise the tower, including the addition of 800 tonnes of lead counterweights to the raised end of the base . </P> <P> The tower and the neighbouring cathedral, baptistery, and cemetery are included in the Piazza del Duomo UNESCO World Heritage Site, which was declared in 1987 . </P> <P> The tower was closed to the public on January 7, 1990, after more than two decades of stabilisation studies and spurred by the abrupt collapse of the Civic Tower of Pavia in 1989 . The bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred meters away . Apartments and houses in the path of the tower were vacated for safety . The solution chosen to prevent the collapse of the tower was to slightly straighten it to a safer angle by removing 38 cubic metres (1,342 cubic feet) of soil from underneath the raised end . The tower was straightened by 45 centimetres (17.7 inches), returning to its 1838 position . After a decade of corrective reconstruction and stabilization efforts, the tower was reopened to the public on December 15, 2001 and was declared stable for at least another 300 years . In total, 70 metric tons (77 short tons) of earth were removed . </P> <P> In May 2008, engineers announced that the tower had been stabilized such that it had stopped moving for the first time in its history . They stated that it would be stable for at least 200 years . </P>

When was the leaning tower of pisa closed