The power of ice is extraordinary, as the Norwegian fjords can attest. But the greatest masterpieces lie buried kilometers deep, and they're made of ice itself, not the rocks on which it sits. Vast complex ice sculptures have been found buried within the Greenland Ice sheet, transforming our understanding of how ice behaves – with as yet unknown implications for how it melts.

Water can stay liquid below 0°C if the pressure is high enough, but will freeze instantly when the pressure is reduced, as Columbia University's Dr Robin Bell demonstrates with this soft drink bottle. 

 

 

The pressure of millions of tonnes of ice above, combined with friction from scraping over rocks, can melt ice at the base, a factor that has forced upward revisions of the rate of ice loss from global warming. In Greenland it is the process of being squeezed up the walls of valleys that reduces the pressure on this supercooled water until it freezes. Bell has revealed just how complicated the layers can become when this refreezing occurs.

Bell first observed this phenomenon in Antarctica but even some fellow glaciologists missed it. Her colleague Kirsty Tinto was helping map the Greenland ice sheet when she noticed something unusual. “When you’re flying over this flat, white landscape people almost fall asleep it’s so boring—layer cake, layer cake, layer cake,” says Tinto. “But then suddenly these things appear on the screen. It’s very exciting. You get a sense of these invisible processes happening underneath.” 

^{Kirsty Tinto. The relatively flat surface of the ice belies what is happening beneath.}

Previously such shapes showing up on radar were taken to be mountain ranges, but Bell recognized them as the result of refreezing. She and Tinto have published their findings in Nature Geoscience, using the Peterman Glacier in north-west Greenland as an example. They found a dozen features the size of skyscrapers being swept to the coast. Two years ago the Petermann calved an iceberg of the size more usually seen in the Antarctic. 

Some of the “basal ice units” the team found are 1100m thick. To map the shapes they used a combination of radar and gravity anomalies compared with what is known about the rock below. However, where the equivalent Antarctic features were found only in the interior, in Greenland they exist right to the edge, interacting with lakes and rivers that fall through crevasses as the surface. Basal melting was found all the way to the giant island's coast, although as one gets closer to the ocean surface melting becomes dominant.

The ice structures are estimated to cover a tenth of northern Greenland. Refreezing releases energy in the interior, warming the ice sheet, changing the structure of the ice column and speeding the flow hundreds of kilometers inland. “We see more of these features where the ice sheet starts to go fast. We think the refreezing process uplifts, distorts and warms the ice above, making it softer and easier to flow,” said Bell.

Commenting in the same edition of Nature Geoscience, University of Texas researcher Joseph MacGregor commented, Overall, these observations suggest that basal freeze-on is a key control on the large-scale flow of Petermann Glacier, a possibility that has not been explored previously,”  

Understanding the shape of the topography at the base of glaciers is essential to the knowing how fast they'll melt, and therefore how rapidly low lying areas will be inundated.