Researchers were surprised to see liquid water coming up through the drill hole as temperatures were below freezing.
Researchers say they have discovered a large reservoir of melt water that sits under the Greenland ice sheet all year round.
The scientists say the water is stored in the air space between particles of ice, similar to the way that fruit juice stays liquid in a slush drink.
The aquifer, which covers an area the size of Ireland, could yield important clues to sea level rise.
The research is published in the journal Nature Geoscience.
The melting of the Greenland ice sheet has been a significant contributor to a rise in sea levels over the past 100 years.
According to the latest report from the Intergovernmental Panel on Climate Change, the ice sheet lost 34 billion tonnes of ice per year between 1992 and 2001 -- but this increased to 215 billion tonnes between 2002 and 2011
Scientists still have many unanswered questions about the direction and speed and ultimate destination of this melted water.
Ice free liquid
This new research finds that a significant amount is stored in partially compacted snow called firn.
In the spring of 2011, researchers drilled deep into this slushy layer and to their surprise, found liquid water flowing back to the surface even though air temperatures were -15 C.
As this was well before the onset of the summer melt, the team concluded the water had persisted in a liquid state through the Greenland winter.
"This discovery was a surprise," said Prof Rick Forster from the University of Utah.
"Instead of the water being stored in the air space between subsurface rock particles, the water is stored in the air space between the ice particles, like the juice in a snow cone."
The scientists have also come up with a rough estimate for the amount of water that is contained in the aquifer which itself covers an area of 70,000 sq km.
The researchers drilled in areas with large accumulations of snow on the surface.
They believe that it holds roughly 140 billion tonnes of water, which is the equivalent to 0.4 mm of sea level rise per year -- about half of what Greenland contributes to the sea every year.
But crucially the scientists don't know the ultimate destination of the water in the reservoir.
"It depends on whether it is currently connected to a system that is draining into the ocean or if it is a bit isolated and completely acting as a storage source without a current connection," said Prof Forster.
"We don't know the answer to this right now. It's massive, it's a new system we haven't seen before - we need to understand it more completely if we are to predict sea level rise."
The scientists say the water is prevented from freezing by the large amounts of snow that fall on the surface of the ice sheet late in the summer.
This insulates the water from the air temperatures which are below freezing, allowing the water to persist as liquid all year long.
Other researchers believe this discovery may help explain disparities between projections of mass loss by climate models and observations from satellites.
"The large mass of liquid water in firn also represents a heat sink that could be playing a role in Greenland's interaction with the climate system," wrote Dr Joel Harper from the University of Montana, in a comment piece published alongside the study.
"As the intensity of surface melt in Greenland increases and expands upwards to the higher elevations that are covered by firn, liquid water storage may play an expanding role in the ice sheet's future response to climate change."
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