Tamu Massif, Earth's Largest Volcano, Lurks Beneath Pacific Ocean 

The world's largest volcano lurks beneath the Pacific Ocean, researchers announced  (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."
Tamu is 400 miles (650 kilometers) wide but only about 2.5 miles (4 km) tall. It erupted for a few million years during the early Cretaceous period, about 144 million years ago, and has been extinct since then, the researchers report.