If you've ever flown from California to Japan, you may have flown over the largest volcano on earth.

You wouldn't have seen it though, because it's deep under water. It's called Tamu Massif and it's comparable to Olympus Mons on Mars,  one of the biggest volcanos that we know about in our solar system.

Up until recently no one had any idea that Massif was a giant volcano, and now that raises the question of how we could miss something so enormous.

William Sager a professor of earth and atmospheric sciences at the University of Houston has the answers. He recently published a paper about Massif in the journal Nature.