The largest volcano on earth is Mauna Loa on Hawai'i Big Island. It is a massive shieldvolcano constructed by countless lava flows. When measured from the base to the top, the pile of lavas measures more than 17,000 m (56,000 ft)!
When measured from the sea floor, Mauna Loa's height is still more than 9,000 m, thus it is also the highest mountain on earth. Mauna Loa in fact is so heavy, that its weight has bent the oceanic crust under the volcano several kilometers downwards into the mantle.
Mauna Loa is one of the Earth's most active volcanoes, with 33 well-documented eruptions in historic times since 1843. Its last eruption was in 1984 and since 2004, Mauna Loa is showing increasing signs of a possible awakening in a not-too-distant future.