The tallest underwater mountain also happens to be the tallest mountain in the world, Mauna Kea. An inactive volcano in Hawaii, Mauna Kea measures 33,500 feet, or 10,210 meters, from its underwater base to its summit.
Mauna Kea is one of Hawaii's five hot-spot volcanoes. It began its growth around 1 million years ago, caused by the Earth's thin crust in the area. Roughly 500,000 years ago, Mauna Kea entered a very active phase, growing quickly as magma pushed up through the Earth's mantle. Mauna Kea is so tall that even though it lies so close to the equator, the summit exhibits evidence of glaciation.