Volcanoes are vents, fissures or openings in the Earth's crust from which hot ashes, gases and magma erupt. On land, volcanoes usually take the form of mountains or hills with one or more volcanic vents. But the vast majority of the world's volcanoes lie beneath the sea along the global oceanic ridges. Around 50 to 60 land-based volcanoes are active in a given year, some posing serious threat to nearby human population.