Although normally thought of as a feature of mountainous regions, landslides can happen almost any place where the ground surface slopes. In fact, some of the largest landslides on Earth occur under water. Suboceanic or submarine landslides can involve the movement of rocks and sediments entirely beneath the sea, or they can begin as partly above-water landslides that later enter the ocean. Like open-air landslides, submarine landslides often strike steep inclines (~10°). Unlike open-air slides, submarine landslides also occur in very slightly dipping terrain (<1°). Many historical and prehistorical landslides have raked the slopes of deep ocean trenches and continental margins where strong earthquakes recur periodically. Seismic shaking probably triggered these slides. Other landslides, however, have been located on seismically quiet continental margins, such as the east and west edges of the Atlantic Ocean, and on the flanks of oceanic island volcanoes in Hawaii and the Canary Islands. Best evidence suggests that the potential for suboceanic landslides exists globally, whether in tectonically active or tectonically inactive regions. A primary hazard of submarine landslides, like their land-bound relatives, is the wasting of human-made structures along their path. The newest research, however, perceives that undersea slope failures present an additional threat—landslide-generated tsunami waves.