The Big Island itself consists of 4 active volcanoes and 1 extinct volcano. The four active volcanoes are Mauna Loa, Mauna Kea, Kilauea and Hualalai. Mauna Loa and Hualalai are overdue for an eruption. About 20-30 miles south of Big Island, 3000 feet below sea level, another island, Loihi, is being built. This volcano has been erupting since 1996, and is the only known erupting submarine volcano in Hawaii. It is believed that one day Loihi may join with its northern sister, the Big Island, to form a larger island, though by then the Big Island might have lost its northern coast. To add to the complexity of the Big Island, there is a sixth volcano slightly northwest of the Big Island, Mahukona. This volcano is considered a submarine volcano, standing at about 900 feet below sea level. It is the oldest of any of the Big Island associated volcanoes, having last erupted about 300,000 years ago.