These features were discovered and explored (1) _______ in the late 1940s. The Lomonosov Ridge, the major ridge, cuts the Arctic Basin (2) _______ in half, extending as a submarine bridge 1,800 km (1,100 mi) (3) _______ Siberia to the northwestern tip of Greenland. Parallel (4) _______ it are two shorter ridges: the Alpha Ridge on the North American (5) _______, defining the Canada and Makarov basins, (6) _______ the Arctic Mid-Ocean Ridge on the Eurasian side, (7) _______ the Nansen and Amundsen basins. The average (8) _______ of the Arctic Ocean is only 1,300 m (4,300 ft) (9) _______ the vast shallow expanses on the continental shelves. The deepest (10) _______ in the Arctic Ocean is 5,450 m (17,880 ft).

(3) ....................

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