Robin George Andrews writes:

On September 16, 2023, the world began to rumble.

A gargantuan rock-ice avalanche tumbled into the deep waters of a fjord in eastern Greenland, unleashing a megatsunami whose initial waves reached a height of 200 meters. The waves scoured the walls of the fjord before flowing into the open sea.

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, 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, 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 — from peculiar magmatic movements to jiggling ice sheets — seemed to make sense.

“If we found no other explanation, we would have gone for sea monster or baby dragons,” joked team member Stephen Hicks, a seismologist at University College London.

Now, as reported today in the journal Science, a team of 68 scientists has managed to identify the source of the world-wobbling hum: a bizarre natural phenomenon, rhythmically hammering on Earth’s surface like a drum. [Continue reading…]