Thankfully, no one was around to experience a 650-foot-high mega tsunami that sprung up close to a cruise ship route on Greenland's east coast last year, but scientists know it happened based in part on seismic waves. A seismic signal showed that the Earth shook over nine consecutive days last September, but "no one had the faintest idea what caused this signal," says Kristian Svennevig, a geologist at the Geological Survey of Denmark and Greenland who set out to explore what happened. A key to solving the mystery was the oscillating signal, with a 92-second interval between peaks, which looked nothing like an earthquake does on seismographs, and remained strong for days on end, per Live Science.
The "completely unprecedented" signal was traced to eastern Greenland, where satellite and ground imagery showed a chunk of mountaintop standing nearly 4,000 feet above Dickson Fjord, part of Northeast Greenland National Park, had collapsed, per CNN. Scientists concluded the glacier at the base of the mountain had melted, unsettling enough rock and ice to fill 10,000 Olympic-sized swimming pools. All of it landed in the waters of the narrow, bendy fjord on Sept. 16, eliciting a 650-foot wave, according to computer simulations. Essentially trapped, the wave sloshed back and forth in the fjord in a phenomenon known as a seiche—hence the oscillating signal.
"We've never seen such a large scale movement of water over such a long period," researcher Stephen Hicks, a seismologist at University College London, tells the BBC. "We only managed to solve this enigma through a huge interdisciplinary and international effort," adds Svennevig, lead author of the study published Thursday in Science, which involved 68 scientists from 41 research institutions in all. A release points out the role of climate change in melting the glacier, adding that more large, destructive landslides could come with further melting of the polar regions. Though no one was hurt, the Greenland tsunami destroyed some $200,000 worth of infrastructure at an Ella Island research station. (Alaska should take note.)