Scientists report in the journal Nature Geoscience that they've uncovered the largest volcano on Earth in the Northwest Pacific Ocean, about 1,000 miles east of Japan. In fact it's one of the largest in the solar system, second only to Olympus Mons on Mars. Scientists have been studying the massive structure for decades, but now are confirming it's a single volcano about the size of New Mexico. It rises about four miles off the sea floor, but doesn't break through the ocean surface. Called Tamu Massif, it hasn't erupted in more than 130 million years, helping to keep its true nature secret.