Semispochnoi Island volcanoes (Image: Wikipedia) A historic eruption was reported in 1873, and at least four others may have occurred in the previous hundred years, but documentation is scant. In December, geologists discovered that a trail of volcanic islands off the coast of the US state may actually be connected to one giant caldera. The cluster of six volcanoes in the Aleutian Islands – known as the Islands of the Four Mountains – may be connected to a much bigger underwater volcano. Scientists analysed seismic activity, gas emissions, gravity measurements and geochemistry in the region surrounding the six stratovolcanoes of Carlisle, Cleveland, Herbert, Kagamil, Tana and Uliag – and their results suggested activity could be traced to a bigger source.