A mud volcano erupted at Diglipur in the Andaman Islands. Also called sedimentary or gas-oil volcanoes, mud volcanoes are mounds formed when mud, water and gases burst out from below the surface, usually in oil-field or hot-spring areas because of methane pressure and sediment compaction. About 1,000 exist worldwide on land and in shallow seas; in India, the only known mud volcano is at Baratang Island in the Andaman and Nicobar Islands.