Volcanoes are mountains that form when magma escapes from the earth's crust through openings or vents. Inside volcanoes is a deep hole that goes down into the earth. There are about 500 active volcanoes on land, including Kilauea volcano in Hawaii, which is more active than any other. Underwater volcanoes can grow large enough to form islands. The Galapagos Islands were formed by underwater volcanoes. Vesuvius volcano in Italy erupted in the past, destroying Pompeii. The ruins of Pompeii were rediscovered over 1,500 years later and have provided information about ancient Roman civilization.