The entire Mediterranean loses three times more water by evaporation than it gains from rainfall and rivers combined. This loss is balanced by a surface inflow from the Atlantic through the Strait of Gibraltar. The inflow continues as an eastward current along the north African coast, giving rise to a counterclockwise circulation in the western Mediterranean. At depth there is a strong undercurrent of outflowing salty water. The flat floors of the Algerian and Balearic basins are underlain by deep sediments. In contrast, the Tyrrhenian Sea contains many seamounts and ridges. A chain of active volcanoes (including Etna, Stromboli, and Vesuvius) is found on the sea’s eastern margin, where the African Plate is subducting beneath the Eurasian Plate. The eastward flow of surface water continues through the Strait of Sicily into the eastern Mediterranean. The narrower Strait of Messina, between Sicily and mainland Italy, is notorious for its whirlpool, possibly the inspiration for the Greek mythological sea monster Charybdis.