Tamu Massif, a “seamount”—submerged mountain—off the coast of Japan, has turned out to be one continuous volcano formation.

William Sager, University of Houston geophysicist and the lead author of a study that Nature Geoscience published online Thursday, said that the seamount is about equal to New Mexico or the British Isles in square miles. Its heft puts it almost on par with Mars’ Olympus Mons, reputed to be this solar system’s largest volcano.

Olympus Mons maintains a clear height advantage. Its summit is 21,000 meters above the Martian surface, while Tamu Massif is only 3,900 meters tall, much of its mass having long ago sunk into the ocean’s crust.