IN 1974-75, an airborne radio-echo survey of ice depths over central East Antarctica led to the discovery of a sub-ice lake of unknown depth and composition, with an area of about 10,000 km(2) and lying beneath similar to 4 km of ice(1). In 1993, altimetric data from satellite measurements(2) provided independent evidence of the lake’s areal extent, thus confirming it to be the largest known sub-ice lake by an order of magnitude, Here we analyse nem altimetric and radio-echo data, along with existing seismic data(3), to show that the lake is deep (mean depth of 125 m or more) and fresh, and that it has an area that exceeds previous estimates by about 50%-dimensions comparable with those of Lake Ontario. We estimate that the residence time of the crater in the lake is of the order of tens of thousands of Sears, and that the mean age of water in the lake, since deposition as surface ice, is about one million years, Regional ice-dynamics can be explained in terms of steady-state ice flow along and over tile lake.