We analyze data acquired by the CryoSat-2 interferometric radar altimeter and demonstrate its novel capability to track topographic features on the Antarctic Ice Sheet. We map the perimeter and depth of a 260 km² surface depression above an Antarctic subglacial lake (SGL) and, in combination with Ice, Cloud and land Elevation Satellite laser altimetry, chart decadal changes in SGL volume. During 2007-2008, between 4.9 and 6.4 km³ of water drained from the SGL, and peak discharge exceeded 160 m³ s ^-1. The flood was twice as large as any previously recorded and equivalent to ~ 10% of the meltwater generated annually beneath the ice sheet. The ice surface has since uplifted at a rate of 5.6 ± 2.8 m yr ^-1. Our study demonstrates the ability of CryoSat-2 to provide detailed maps of ice sheet topography, its potential to accurately measure SGL drainage events, and the contribution it can make to understanding water flow beneath Antarctica. Key Points Assessment of novel CryoSat-2 interferometric altimetry over land ice Demonstration of interferometric capability to track off-nadir topography Mapping of largest Antarctic subglacial lake drainage event observed to date.