Ross Sea, arm of the Pacific Ocean, Antarctica, between Victoria Land and Marie Byrd Land. It was discovered in 1841 by Sir James Clark Ross, a British explorer. Ross Island with Mt. Erebus, an active volcano, is in the western part of the sea; Roosevelt Island is in the east. The Ross Sea's southern extension is the Ross Ice Shelf, a great frozen area whose 400-mi (644-km) seaward side is the source of huge icebergs. The Bay of Whales, the ice shelf's best known inlet, lasted for c.50 years and was the site of Norwegian explorer Roald Amundsen's base for his trek to the South Pole in 1911; Little America, a U.S. base, was located nearby. McMurdo Sound, on the western side of Ross Sea, is usually free of pack ice in late summer; it has been the most important staging point for exploration and scientific investigation.