The largest volcano on earth is not on land - it's underwater, reports Nature World News.

The Tamu Massif is around 60 times larger than what was previously thought to be the largest volcano on Earth, Mauna Loa in Hawaii, says NBC News. Tamu Massif measures 120,000 square miles.

Its humungous size is comparable to the size of New Mexico or to the British Isles. It is also only 25% smaller than the giant mountain on Mars, Olympus Mons. Olympus Mons is the biggest volcano in the Earth's solar system, according to lead study author and University of Houston geologist, William Sager.

Although wide at 650 kilometers, Tamu Massif is quite low. Its height only measures 4 kilometers making the slopes very shallow.

"If you were standing on this thing, you would have a difficult time telling which way was downhill," said Sager.

The massive volcano is located in the Pacific Ocean and is under 6,500 feet of water. It lies 1,000 miles off the east coast of Japan.

The megavolcano is reportedly millions of years old but has been inactive ever since it erupted 144 million years ago during the Cretaceous period, says NBC News.

Tamu Massif was initially thought of as a part of Shatsky Rise. Shatsky Rise is an oceanic plateau located in the northwestern part of the Pacific Ocean. Like volcanoes, oceanic plateaus also have mounds of lava though the origins of these piles of lava are still under study.

In 2010 and 2012, the research vessel "Marcus G. Langseth" was set to sail over Tamu Massif in hopes of finding more information about the oceanic plateau. Much to the researchers' surprise, findings show that the mound's lava flows all come from a central magma vent, says Nature.

Geologist Scott Bryan, from Queensland University of Technology in Brisbane, Australia thinks that not all of Tamu may have come from a central magma vent, but rather from separate sources that the research vessels' seismic waves could not penetrate.

Nonetheless, the discovery of Tamu Massif is one great step forward in understanding how massive volcanoes come about.

"Its shape is different from any other sub-marine volcano found on Earth, and it's very possible it can give us some clues about how massive volcanoes can form. An immense amount of magma came from the center, and this magma had to have come from the Earth's mantle. So this is important information for geologists trying to understand how the Earth's interior works," said Sager.