Even for this avalanche-prone corner of Greenland, the collapse and subsequent megatsunami were shocking for their speed and ferocity. But what followed was considerably stranger. The collapse trigged a monotonous growl that kept going for nine days straight. It was strong enough to be detected by seismometers all around the world. And yet, when the Danish military visited the scene, they found nothing amiss. "This is crazy," said Kristian Svennevig(opens a new tab), a geologist at the Geological Survey of Denmark and Greenland and a member of a team that investigated the anomaly. It was like nothing anyone had ever seen. "Immediately, there was a massive amount of researchers joining forces," said Finn Løvholt(opens a new tab), a tsunami and landslide researcher at the Norwegian Geotechnical Institute in Oslo. The team quickly grew to include experts from 15 different countries. The Danish navy offered their help. But nothing anyone suggested as an underlying cause ?"