The arctic ice cap averages a mere 3 meters thick. Its thinness is determined by the fact that it rests on liquid water that is above the melting point (-2.7 C for saline seawater) and is constantly being melted from underneath as snow and ice accumulates on the surface. It is therefore impossible for the core to be as long as is shown here, and equally impossible, for the same reason, for any ice from an arctic core to be more than a few years old. The antarctic ice cap, however, rests mostly on land, and in places reaches thicknesses of up to five kilometers and has an average thickness of 2.5 km. (Despite the lower temperatures, there are more than 140 bodies of liquid water known to exist under the antarctic ice cap; the largest, Lake Vostok, is under 4 km of ice.)