The Greenland ice sheet rests on rock. Because the ice sheet is very large and thick it is difficult to know the shape of the rock below it. Our information comes from planes that shoot a radar beam through the ice which bounces off the rock and tells us the height of the rock surface. This has shown us that a valley appears to extend from the north coast all the way into the center of Greenland. However because the rock height information is not complete we have to adjust it to create a valley without blocks in it. After this we run a computer simulation of Greenland to see what changes the valley causes. This simulation shows that this valley contains liquid water all along its length that comes from melting ice. The valley takes a path that goes from where the ice above it is thicker and heavier to where it is thinner and lighter and this should push the water along the valley towards the coast. This makes us think that there may be a river over 1600 km long running in darkness under the ice. We call it the Dark River.