The most massive volcano on Earth, with a footprint the size of New Mexico, crouches in the dark depths of the western Pacific Ocean. The base of the basaltic mound, Tamu Massif, may rival in area the largest known volcano in the solar system: Mars’ Olympus Mons. In 2010 and 2012, a team led by oceanographer William Sager, then at Texas A&M University, bounced sound waves off the deep-sea mountain to measure its size. The researchers report September 8 in Nature Geoscience that Tamu Massif forms a broad, rounded dome rising about 4 kilometers from the seafloor and stretching 450 by 650 kilometers across. Its hollowed peak lies beneath 2 kilo-meters of water. Core samples extracted from the volcano’s slopes show that, during its prime 145 million years ago, the ancient mound spewed lava sheets 23 meters thick.