Geophysicist Eric Geist explains what causes tsunamis and the many unanswered scientific questions about these giant waves. Geist, who is based at the USGS Menlo Park Science Center, studies the mechanics of tsunamis and the mechanics of how they can be generated by earthquakes on the ocean floor. For more information about natural hazards, check out the July 2017 special issue of Physics World magazine. See http://ow.ly/RHx230dxJMP