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The immense and forbidding Southern Ocean is famous for howling gales and devilish swells that have tested mariners for centuries. But its true strength lies beneath the waves. The ocean’s dominant feature, extending up to two miles deep and as much as 1,200 miles wide, is the Antarctic Circumpolar Current, by far the largest current in the world. It is the world’s climate engine, and it has kept the world from warming even more by drawing deep water from the Atlantic, Pacific and Indian oceans, much of which has been submerged for hundreds of years, and pulling it to the surface. There, it exchanges heat and carbon dioxide with the atmosphere before being dispatched again on its eternal round trip.