The collapse of an ice formation started a wave that ping-ponged between two sides of a fjord in Greenland for nine days, growing to a massive 650 feet before collapsing, NBC News reports. The wave, which happened last September, was not seen, but instead identified through seismic records and geological scars.
The ice and rock fall caused a seiche, a standing wave, that bounced off the walls of Dickson Fjord, causing seismographs around the world to record a low-level hum. A popular sightseeing location for cruise ships, two ships had passed by two days before the hum started. One cruise ship passed after the hum started, reporting a nearby island appeared destroyed; scientists say it was hit by a 13-foot tsunami that escaped the fjord.