This is an Inside Science story.
It's a scene from the "Ice Age" movies -- a vast freshwater lake, created by melting ice, breaks through a thin retaining wall of ice, releasing a huge wall of water.
In this case, the dramatic event happened repeatedly, in what is now Greenland. New research suggests that such "outburst floods" from lakes of glacial meltwater may have carved out one of the largest networks of canyons on Earth.
For at least a million years, these canyons, which cut through central and northern Greenland, have been covered by an ice sheet almost 2 miles thick.
If the ice sheet wasn't there, Greenland's canyons would be one of Earth's most visible scars: The largest canyon is about 800 meters deep and, at more than 450 miles long, it's almost twice as long as Arizona's Grand Canyon.
"We would definitely be able to see it from space," said glaciologist Benjamin Keisling.
Keisling, now at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, is the lead author of a new study of Greenland's hidden canyons published in the journal Geology.
The research by Keisling and his colleagues suggests the canyons formed from lakes of glacial meltwater, which ice dams held back until the dams also melted and broke. The sudden release of water then gouged out the canyons from the bedrock.
Scientists long suspected there were depressions in the bedrock below Greenland's ice sheet, but they had no idea of their scale.
The true size of the canyon network was revealed in 2013, by combining years of ice-penetrating radar surveys from British, German and NASA researchers.
"It became pretty clear that this was a continuous feature," said physicist Jonathan Bamber of the University of Bristol in the U.K., who revealed the extent of the hidden canyon network but who was not involved in the new research.
His study showed the canyon network extends from the northern coast to the center of Greenland.
But Bamber and his colleagues were surprised to find that the canyons did not seem to have been formed by the ice sheet above them.
Instead, their shape indicated that rivers formed the gouges. Initially, they believed that it could have happened several million years ago, when Greenland was almost balmy and before the growth of the ice sheet.
The new research, however, suggests Greenland's canyon network formed almost in step with the ice sheet.
The lack of direct geological evidence from the deep bedrock canyons means the researchers can't be sure when it happened.
But they think it took place during cycles of climate cooling and warming near the end of the Pliocene Epoch and the start of the Pleistocene ice age about 2.7 million years ago.
Canyons are formed by two main geological processes. One process is similar to the one that created the Great Lakes more than 14,000 years ago, when glaciers carved giant depressions into the bedrock.
Alternatively, outburst floods gouge out canyons when large volumes of water suddenly escape the reservoirs that held them -- such as lakes of glacial meltwater dammed up by barriers of ice.
Such a process of outburst flooding occurred in the Columbia River Gorge during the melting of the Laurentide ice sheet, which covered most of North America until about 20,000 years ago, Keisling said.
In the case of Greenland, his simulations suggest the trapped meltwater lakes were truly huge -- perhaps hundreds of kilometers across. When they burst through their ice dams, as much as 6 cubic miles of water could have flowed to the sea.
And it may have happened hundreds of times, he said, as cycles of cooling and warming regrew the ice sheet and melted it again over hundreds of thousands of years.
The volumes of water released by the glacial outburst floods may have been so large that they changed the ocean circulation around Greenland and affected the regional or global climate at times, Keisling said.
The hidden canyons could also have implications for the stability of the Greenland ice sheet today.
Some scientists worry that Greenland's ice sheet is melting at an alarming rate, and could be a harbinger of global climate changes.
Keisling suspects the hidden canyons neither slow nor speed up the current melting, but "it is important that we understand how the ice sheet has changed in the past, and how that relates to what might happen in the future."
Bamber agrees. He finds the new research "fascinating" and suggests the canyons still influence how the ice sheet behaves today.
Recent studies suggest some of the canyons may be filled with melting water, which could affect how ice moves from the center of Greenland toward its margins. "They're not major impacts, but it certainly it does have some impact," Bamber said.
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