Two supervolcanoes, the Campi Flegrei in Italy and the Long Valley Caldera in California, are showing signs of unrest. However, scientists say there is no immediate threat of an eruption.
Supervolcanoes are capable of huge explosions, but most eruptions are smaller. The Campi Flegrei volcano last erupted in 1538. The Long Valley Caldera last erupted in 1915. Scientists are monitoring both volcanoes closely.