Long Valley, California, has long defined the ‘super-eruption.’ About 765,000 years ago, a pool of molten rock exploded into the sky. Within one nightmarish week, 760 cubic kilometers of lava and ash spewed out in the kind of volcanic cataclysm we hope never to witness. A new study shows that the giant body of magma — molten rock — at Long Valley was much cooler before the eruption than previously thought.