Skimming the surface of underwater landslides.

Around 8,200 years ago, a 180-mile-wide section of the ocean floor off the coast of Norway reached a tipping point. Several thousand years prior, ice streams from melting glaciers had been carrying trillions of tons of sediment to the ocean. Over time, the sediment had accumulated at the edge of the continental shelf, where the shallower ocean floor around Scandinavia plunges toward the depths of the Norwegian Sea. But finally, the underwater build-up became unstable; something set it off, perhaps an earthquake. An area of the seafloor roughly the size of Maine started collapsing, sliding down the shelf at speeds reaching more than 40 miles per hour. The vast slide continued for several hours, with material coming to rest as far as 500 miles downstream, halfway to Greenland. The massive movement also riled up the ocean, sending tsunamis racing toward a handful of northern European shores, eventually flooding areas as high as 20 meters above sea level in some places. “It’s hard to even comprehend the size of a landslide like this,” says geologist Peter Talling of the United Kingdom’s National Oceanography Centre. “A single landslide can move more sediment than all the world’s rivers move in a year.” The Storegga Landslide off Norway isn’t the only one of its kind, nor the biggest. Geological evidence reveals that submarine landslides as large or larger than Storegga occurred off New Zealand and South Africa in the distant past. In the last century alone, much smaller submarine landslides have generated tsunamis off Alaska, Venezuela, Papua New Guinea, Newfoundland, and the Mississippi Delta. Some recently published evidence even suggests that the N E W S F E A T U R E