http://www.livescience.com/39447-biggest-volcano-earth-found.html

The world's largest volcano lurks beneath the Pacific Ocean, researchers announced today (Sept. 5) in the journal Nature Geoscience.

Called the Tamu Massif, the enormous mound dwarfs the previous record holder, Hawaii's Mauna Loa, and is only 25 percent smaller than Olympus Mons on Mars, the biggest volcano in Earth's solar system, said William Sager, lead study author and a geologist at the University of Houston.

"We think this is a class of volcano that hasn't been recognized before," Sager said. "The slopes are very shallow. If you were standing on this thing, you would have a difficult time telling which way was downhill."

Rest at link, plus imagery.

Brian