It measures as large as New Mexico and it has lain dormant at the bottom of the Pacific Ocean for 140 million years.

A volcano that size may seem hard to miss, but researchers have only just discovered the truth behind Earth’s largest volcano, which lies about a thousand miles east of Japan.

Geophysicist and the study’s lead author, William W. Sager of the University of Houston, has studied the shield volcano, named Tamu Massif, for two decades, the Washington Post reports. It took Sager and his colleagues all this time to gather the evidence needed to prove that what they thought was a series of volcanoes was actually one single, colossal volcano.

Tamu Massif has long been thought to be one of three underwater mountains that comprised the range called the Shatsky Rise. However, the team’s research has concluded that this is not the case.

"We know that it is a single immense volcano constructed from massive lava flows that emanated from the center of the volcano to form a broad, shield-like shape,” Sager told Nature World News. “Before now, we didn't know this because oceanic plateaus are huge features hidden beneath the sea. They have found a good place to hide."

In width, Tamu Massif even rivals the size of the Solar System’s largest volcano, Olympus Mons of Mars, according to a report by nature.com. The volcano proves that Earth is capable of creating supervolcanoes.

The research study on Tamu Massif was published online in the journal Nature Geoscience on Thursday.