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Mauna Loa is an active shield volcano in the Hawaiian Islands, one of five volcanoes that form the Island of Hawaii. Mauna Loa is Earth's largest volcano, with a volume estimated at approximately 18,000 cubic miles (75,000 km³), although its peak is about 36 m (120 ft) lower than that of its neighbor, Mauna Kea. Lava eruptions from Mauna Loa are very fluid and the volcano has extremely shallow slopes as a result. The volcano has probably been erupting for at least 700,000 years and may have emerged from the sea about 400,000 years ago. Its magma comes from a hotspot in the Earth's mantle far beneath the island that has been responsible for the creation of the Hawaiian island chain for tens of million of years. The slow drift of the Pacific Plate will eventually carry the volcano away from the hotspot, and the volcano will thus become extinct within 500,000 to one million years from now. The first recorded summiting of Mauna Loa was in 1794 by naturalist Archibald Menzies, then-Lieutenant Joseph Baker, and two others. Mauna Loa's most recent eruption occurred from March 24, 1984 to April 15, 1984. In view of the hazards it poses to population centers, Mauna Loa is part of the Decade Volcanoes program, which encourages studies of the most dangerous volcanoes. Mauna Loa has been intensively monitored by the Hawaiian Volcano Observatory (HVO) since 1912. Observations of the atmosphere are undertaken at the Mauna Loa Observatory, and of the Sun at the Mauna Loa Solar Observatory, both located near its summit.