A broad, 4-kilometer-tall feature on the seafloor about 1500 kilometers east of Japan is the world’s largest volcano, a new analysis suggests. At its tallest point, Tamu Massif (at lower left and center in main image; oblique view in inset) lies more than 2 km below the ocean’s surface. Unlike most volcanic seamounts, which are steep and typically no more than a few tens of kilometers across, the gently sloping Tamu Massif covers 310,000 square kilometers—about the same as the British Isles, or the base of Mars’s Olympus Mons, the solar system’s largest known volcano. (Its base is shown in dark purple at lower right, for comparison.) The massif’s slopes are exceptionally shallow, often less than 1°, thanks to lava that flowed freely before hardening. Researchers think the Tamu Massif is a single volcano because rock samples (labeled dots) have similar chemistry, and seismic surveys show that broad layers of rock emanate from the center of the feature. Today, Tamu Massif sits far from the edge of the Pacific tectonic plate and is presumed dead, but 145 million years ago the caldera plumbed the intersection of three tectonic plates, the researchers note today in Nature Geoscience. They haven’t finished dating rock samples drilled from the peak, but it’s possible that the entire seamount could have been formed in a million years or less.