Civita di Bagnoregio, Italy

The town is noted for its striking position atop a plateau of friable volcanic tuff overlooking the Tiber river valley, in constant danger of destruction as its edges fall off, leaving the buildings built on the plateau to crumble. As of 2004, there are plans to reinforce the plateau with steel rods to prevent further geological damage. The city is also much admired for its architecture, some spanning several hundred years. Civita di Bagnoregio owes much of its unaltered condition to its relative isolation: the town was able to withstand most intrusions of modernity as well as the destruction brought by two world wars. The population today varies from about 12 people in winter to over 100 in the summer.
The town was placed on the World Monuments Fund's 2006 Watch List of the 100 Most Endangered Sites, due to the threats it faces from erosion and unregulated tourism.[source]