SEATTLE, WASHINGTON—Some 630,000 years ago, the supervolcano beneath Yellowstone National Park in Wyoming recorded its last catastrophic eruption, forming a caldera that nearly spans the park’s width and belching a thick layer of ash, or tephra, across North America. But rather than a single event, Yellowstone may have erupted twice in a span of 270 years, new evidence from mud cores discovered off the coast of Santa Barbara, California, indicates. The cores, presented here today at the annual meeting of the Geological Society of America, were captured at the farthest extent of the ash’s reach, recorded as wisps of tephra in finely sedimented, ancient mud uplifted near the ocean floor. Most evidence of the Yellowstone eruption (the park’s Grand Prismatic geyser is pictured) is found on land in thick layers of compacted, weathered rock, which could have easily hidden the dual eruption, the researchers say. Both tephra layers also coincide with a stark temperature decline of 3°C, according to the core’s records of oxygen isotopes and fossilized plankton, with each episode lasting 100 years or more. If confirmed, the research could indicate that Yellowstone can recharge its eruptions much more quickly than typically thought—and that traditional views of volcanic winter, the period of cooling caused by a volcano’s reflective droplets and ash, fail to explain how a century of cooling could follow the eruptions.