A 3-D image of Tamu Massif on the sea floor. Photo: Will Sager

A massive volcano the size of New Mexico or the British Isles lurks deep beneath the Pacific, about 1,000 miles east off the coast of Japan. Called the Tamu Massif, scientists just confirmed that it is not only the world’s largest volcano (sorry, Manua Loa) but also one of the largest documented volcanoes in the solar system.

Researchers began studying the Tamu Massif, which is part of an underwater mountain range, about 20 years ago. But until now, they couldn’t determine whether it was a single giant or a cluster of multiple smaller volcanoes. A team from Texas A&M University (“Tamu”—get it?) confirmed the Tamu Massif was a single volcanic entity by studying its past patterns of lava flows and analyzing geochemical samples from the volcano.

National Geographic describes what we know about the volcano:

Tamu Massif is a rounded dome that measures about 280 by 400 miles (450 by 650 kilometers), or more than 100,000 square miles. Its top lies about 6,500 feet (about 2,000 meters) below the ocean surface, while the base extends down to about 4 miles (6.4 kilometers) deep.

Made of basalt, Tamu Massif is the oldest and largest feature of an oceanic plateau called the Shatsky Rise in the northwestern Pacific Ocean. The total area of the rise is similar to Japan or California.

Luckily for us, the volcano was only active for a few million years, NatGeo points out, going “extinct” about 145 million years ago.

More from Smithsonian.com:

Underwater Volcano
Volcano Obsession

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An Underwater Volcano the Size of New Mexico Is the World’s Largest