Atlantic Ocean World's second-largest ocean stretching from the Arctic Circle in the n to the Antarctic Ocean in the s. Its name derives from the Atlas Mountains, which, for the ancient Greeks, marked the western boundary between the known and the unknown world. Its most striking feature is the Mid-Atlantic Ridge, which runs n–s for its entire length. At the crest, the ridge is cleft by a deep rift valley which is frequently offset by e–w transform faults. The age of the crust increases with distance from the central rift, and so there is little doubt that the rift has evolved by seafloor spreading and is associated with the movement of the Americas away from Europe and Africa at a rate of 2–4cm (0.8–1.6in) a year. The average depth of the Atlantic is 3700m (12,100ft). The greatest known depth is the Milwaukee Deep in the Puerto Rico Trench, at 8650m (28,370ft). The n clockwise gyre is dominated by the fast-flowing Gulf Stream, travelling at speeds of up to 130km (80mi) a day, and forming the w boundary current of the gyre. The s anti-clockwise gyre is atypical in having a weak western boundary current, the Brazil Current. Apart from oil (found mainly in the Gulf of Guinea), sand and gravel are the most important minerals from the Atlantic. The world's largest single offshore mining operation is located at Ocean Cay on the Grand Bahamas bank, where calcium carbonate is extracted in the form of aragonite. Valuable diamond deposits occur off the coast of Namibia. The North Atlantic contains the most valuable fishing grounds in the world, namely the cod fisheries around Iceland, s Greenland and the Grand Banks of Newfoundland. Area: 82 million sq km (32 million sq mi).