Deep beneath Japan’s mostly submerged Kikai caldera, scientists have discovered a vast magma reservoir that appears to be filling again. The volcano produced the largest eruption of the Holocene 7,300 years ago, and seismic surveys now show that fresh magma has likely entered the same underground system responsible for that catastrophe.

The discovery, led by researchers at Kobe University, could improve understanding of enormous caldera systems such as Yellowstone in the United States and Toba in Indonesia. It may also help scientists identify the underground changes that occur before future giant eruptions.