When an interlacing geometric pattern emerged on one of Iceland's most famous natural landmarks last month, locals were more than a little puzzled.

Image: Einar Á.E. Sæmundsen/used with permission

The icy Etch-a-Sketch appeared on Lake Þingvallavatn in Þingvellir National Park (ÞNP), which sits just east of Reykjavík. Until now, nothing like it has ever been documented here. 

Perplexed by the sight, many residents turned to the park's staff for an explanation – but not before offering up a few guesses of their own. Toboggan-riding child prankster? Drone pilot? Intoxicated snowmobile driver? Skidding wildlife? The problem with all of these (creative!) suggestions is that the ice on the lake is too thin and unstable to support any weight. 

Image: Einar Á.E. Sæmundsen/used with permission

Like the Far North version of crop circles, the shapes – which span an impressive 1.25 miles (2km) – also inspired a number of extraterrestrial origin stories since photos of them were posted online.

"Maybe it's Manchester code," wrote one commenter.

"They're not using our same alphabet, same language. If it's aliens, and it just said, 'We are here – Signed E.T.' everyone would think it was a hoax anyways," added another. 

While we're sorry to break up the #TeamAlien fun, there is a logical explanation here – and it's one that eluded scientists and polar explorers for over 50 years. The secret to unravelling this mystery lies in the ice itself. 

Image: Lusilier/Wikimedia Commons

When ice sheets collide, they usually pulverise each other into a field of frozen rubble. But under just the right conditions, extremely thin ice will fracture and overlap in a zipper-like formation known as "finger rafting". You can mimic the process by holding your hands up in front of you, fingers slightly apart, and then allowing your fingers to interlace. Just like your digits, the "fingers" of each ice sheet alternate: ducking over or under each other in a repeated pattern.

"The phenomenon is quite well known," ÞNP staff told us. "It had, however, not been seen before at Lake Þingvallavat. Not by any scientist, not by any farmer. Of course, that does not rule out the possibility it has formed before but was just not [observed]. If so, then it did not happen on one of the several known ice-breaking points on the lake."

In either case, the team suspects these runaway right angles will show up more often in years to come. Rising temperatures in the region mean ice cover has thinned dramatically over the past decade. A lake whose surface was once solid from January well into spring, now sees only a film of pliant, rafting slush. 


Top header image: Jim Flanagan/Flickr