The polar ice caps are important components of the climate system. A volume of ice representing a 58 m change in sea level elevation is stored on the Antarctic continent. The ice, which forms from the accumulation of snowfall, behaves like a viscous fluid that flows under its own weight. The Antarctic ice cap is unique in that it rests on a bedrock with large areas below sea level and is surrounded by large ice shelves which, due to water pressure, float on the oceans. This particular configuration is subject to instability where a small disturbance, such as increased melting under the floating shelves, could lead to a significant and irreversible retreat of the ice sheet. Recent observations have shown a significant acceleration of some of the glaciers draining the ice-sheet, particularly in the Amundsen Sea area, suggesting that this mechanism may have been initiated. IGE has been co-developing the Elmer/ Ice ice-sheet model for several years. It is mainly the initial and boundary conditions that cause difficulties. For example, the conditions at the base are inaccessible to measurement, yet the resistance of the base to ice sliding can vary over several orders of magnitude depending on the type of substrate and the presence of liquid water. The result is an extremely heterogeneous flow where the majority of the ice is drained by outflow glaciers, true rivers of ice within the ice-sheet, which can flow at a rate of a few kilometres per year as illustrated in the Figure. An adaptive mesh and data assimilation methods allow us to capture the flow velocities observed by satellites. Thanks to GENCI resources, we have shown with our partners in the European TiPACCS project that the ice-sheet is currently in a stable configuration in the face of small perturbations.