Subduction zones create the largest, most destructive earthquakes on the planet, but many mysteries remain about the plate tectonic boundaries where these quakes originate. One such boundary lies deep beneath the seafloor off Alaska—one of the most seismically active zones in the world. This area has produced fatal earthquakes and tsunamis similar to the recent one in Japan. In 1964, the second largest quake ever recorded happened here, and other parts of the fault may be building energy for another event. Lamont-Doherty seismologist Donna Shillington describes how seismic data collected last summer aboard the Marcus G. Langseth is helping to understand the Alaska subduction zone and its capacity to produce big earthquakes.