FIGURE 2. The dimensions and topographic setting of Lake Vostok.

a, ERS-1 altimetry of the Antarctic ice sheet between ridge B and dome C. The location of Lake Vostok can be identified from the anomalous flat ice-surface region. The contour interval is 10 m. SPRI (Scott Polar Research Institute) RES flight lines and the location of all known subglacial lakes around Lake Vostok (shown as black squares) are provided. Arrows denote the direction of surface flow of ice over Lake Vostok calculated from InSAR^11,12. b, Cross-section from north to south along the 200 km length of the lake. c, Cross-section from west to east along the 50 km width of the lake. The depth of Lake Vostok can be estimated by: (1) seismic information¹⁵, which has revealed a water depth of over 500 m beneath Vostok station and 1000 m to the north of the station; (2) a side-wall bedrock gradient adjacent to the lake of 0.1, which indicates several hundred metres of water depth in the centre of the lake; (3) radiowave reflections from the lake floor, showing the water depth to be between 10 and 20 m in the north of the lake; and (4) bedrock 'islands' measured by RES.