Mount Teide. Spain

The world's third largest volcano (from its base), Mount Teide, is located on Tenerife, the Canary Islands. Although Teide is currently dormant, further eruptions are possible in the near future, including the risk of pyroclastic flows and surges similar to those that occurred at Merapi or Mount Vesuvius in Italy. Due to Teide's proximity to several large towns and resorts, the mount was designated one of the Decade Volcanoes by the International Association of Volcanology and Chemistry, with the implication that it's currently one of the world's most dangerous volcanoes.