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A valley longer than the Grand Canyon that is hidden under the Greenland ice sheet may carry flowing liquid water, according to a new analysis of the valley鈥檚 shape. The water could affect the movement of the Greenland ice sheet, causing parts of it to move faster or slower towards the sea.

The hidden canyon was first described in 2013 by glaciologist Jonathan Bamber of the University of Bristol in the UK and his colleagues. The valley runs for at least 750 kilometres, stretching from the centre of Greenland to its northern coast. By comparison, the Grand Canyon is just 446 km long 鈥� although much of it is deeper than the Greenland canyon.

However, the map of the canyon was incomplete, because it was obtained by flying planes carrying ice-penetrating radar in straight lines over Greenland. 鈥淵ou can imagine it goes over a valley, and it gets a dip, and it sees the valley,鈥� says Christopher Chambers of Hokkaido University in Sapporo, Japan.

Bamber鈥檚 team had to fill in the gaps between the data the planes had gathered. The statistical method they used did this by taking average bedrock heights from the surrounding area. This made it look like the valley was blocked at several points. 鈥淥ur reconstruction of the bed cannot make it continuous,鈥� says Bamber. 鈥淭hat鈥檚 purely an artefact of the sampling.鈥� Bamber and his team understood this and stated in 2013 that the valley is probably continuous along its entire length, and could have some water flowing along its base.

Chambers and his colleagues have developed now developed a 3D reconstruction of the valley that fills in the gaps based on the canyon rather than the surrounding bedrock. This removed the artefactual blockages. 鈥淲e get this water pathway that links up and goes all the way down towards the coast,鈥� he says.

That implies there could be a trickle of liquid water along the canyon floor. 鈥淭he base of the ice sheet is melting very, very slowly,鈥� says Chambers.

The liquid water could potentially affect the stability of the ice sheet, but in complex ways. If there is consistently a thin layer of water in the Canyon, that would act as a lubricant and allow the ice to slide more quickly into the sea. However, a rapid flow down the canyon would take this water away, slowing down the ice.

鈥淚t鈥檚 a potential problem for the simulations of the ice sheets into the future, because we need to know where the water is going,鈥� says Chambers.

The Greenland ice sheet is threatened by climate change, as warmer temperatures are causing the ice to melt and pushing it towards an irreversible tipping point. The destabilised ice sheet is contributing to rising seas. Understanding what is happening underneath the ice will be key to predicting how rapidly it shrinks.