Scientists have uncovered a hidden weakness beneath the Pacific Ocean that helps explain why Japan’s catastrophic 2011 earthquake and tsunami became so devastating. By drilling deeper into the seafloor than ever before, researchers discovered a thin, slippery layer of ancient clay that allowed the massive rupture to race all the way to the ocean floor, triggering enormous seafloor movement and a powerful tsunami.

Researchers have uncovered a hidden feature beneath the Pacific Ocean that helps explain why Japan’s devastating 2011 earthquake and tsunami became so destructive. The discovery also offers new clues that could improve forecasts of future megaquakes and tsunamis.