Scientists have discovered a frozen underworld beneath the ice sheet covering northern Greenland.
The landscape, a vast expanse of warped shapes including some as tall as skyscrapers, was found using ice-penetrating radar loaded aboard survey flights by US space agency Nasa.
The findings and the first images of the frozen world more than 1.5km below the surface of the ice sheet were published on Sunday in the journal Nature Geoscience.
Scientists said the findings could deepen understanding of how the ice sheets of Greenland and Antarctica respond to climate change.
"We see more of these features where the ice sheet starts to go fast," said Dr Robin Bell, the study's lead author and a geophysicist at Columbia University's Lamont Doherty Earth Observatory. "We think the refreezing process uplifts, distorts and warms the ice above, making it softer and easier to flow."
Until recently, scientists studying the Greenland ice sheet for evidence of change under global warming had thought the shapes they discerned beneath the ice sheet were mountain ranges.
But with new gravity sensors and radar operating from Nasa's airborne surveys of the ice sheet over the last 20 years, scientists eventually concluded the formations were ice - not rock.
The formations were caused by the melting and subsequent refreezing of water at the bottom of the ice sheet, and the scientists said they were initially stunned by the results.
"They simply look spectacular," said Dr Kirsty Tinto, a geophysicist at Lamont-Doherty. "Everything was just flat parallel lines. That is how ice is supposed to be. But here it is breaking all the rules.
"You get these crazy, folded, distorted, overturned, undulating things at the bottom of the ice, and they are the size of skyscrapers," Tinto said.
The structures - some measuring up to a kilometre thick - cover about 10 per cent of the surveyed areas of northern Greenland.
About a dozen were found around Petermann Glacier, which has been changing rapidly and two years ago calved an iceberg twice the size of Manhattan.
The melting and refreezing at the bottom of the ice sheet has been under way for hundreds of thousands of years.
Scientists were aware of the process, but until now they did not realise that the meltwater was refreezing into complex formations.