One of the most recently troubling calderas in the world is the 150-square-mile Aira caldera in southern Japan, on the edge of which sits the city of Kagoshima.
After a century of peace, the Sakura-jima volcano, which forms part of the Aira caldera, awoke on Jan. 10, 1914, and gave local residents two days' notice of its intentions by letting loose hundreds of earthquakes.
On Jan. 12, after 23,000 people and their farm animals living on its flanks were evacuated, Sakura-jima erupted with ash, steam and lava. It was not really a super eruption, but it taught people a lot about how volcanoes erupt.
There was another eruption in 1946, and since 1955 Sakura-jima has had hundreds of small eruptions every year. The biggest eruptions, however, took place 22,000 years ago when 14 cubic miles of material burped out of the ground and formed the Aira caldera, which is now largely Kagoshima Bay. That is equal to about 50 Mount St. Helens eruptions.
