Two small, guano-covered islands that peek above the waves in the central North Pacific Ocean are merely the tips of our planet’s largest single volcano, new research reveals.

Pūhāhonu—Hawaiian for “turtle surfacing for air”—lies about 1100 kilometers northwest of Honolulu. It is a shield volcano—a broad dome that rises about 4500 meters from the sea floor from a single source of molten rock. In an analysis reported this month in Earth and Planetary Science Letters, researchers estimate that Pūhāhonu contains approximately 150,000 cubic kilometers of rock, based on a 2014 sonar survey.

But only one-third of that volume is exposed above the sea floor; the rest is buried beneath a ring of debris, broken coral, and other material that has eroded from the peak. Pūhāhonu is so heavy, researchers note, that it has caused Earth’s crust nearby—and thus the volcano itself—to sink hundreds of meters over millions of years.

From sea floor to peak, Mauna Loa, on Hawaii’s Big Island, is the tallest shield volcano on Earth. But it’s nowhere near as big as Pūhāhonu; a 2013 study estimates Mauna Loa’s volume at 83,000 cubic kilometers. The Tamu Massif, a 4-kilometer-tall volcanic feature the size of the British Isles on the sea floor east of Japan, contains almost 7 million cubic kilometers of material and was once thought to be the world’s largest shield volcano. But now it is believed to have formed along a midocean ridge rather than over a single source of magma. That makes Pūhāhonu the world champion shield volcano … for now.

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