Seamounts are submarine mountains rising at least 1,000 m (3,300 ft) above their surroundings; they are nearly always volcanoes. Some seamounts called tablemounts or guyots have flat tops at depths down to 2,500 m (1.5 mies). These tops are often too large to be explained as ancient craters filled to the rim by sediments. Thus it was proposed that guyots were volcanoes above sea- level (A) which, after extinction, were worn flat by waves (B) and which then sunk as the sea level rose or as the seabed subsided (C). This has been confirmed by the presence of beach pebbles.