Radio-echo sounding reveals a 10 km-long lake beneath ~3.4 km of ice near the Ellsworth Mountains in West Antarctica, 20 km from the ice divide. Subglacial Lake Ellsworth is located within a distinct topographic hollow, which is ~1.5 km deeper than the surrounding bed. Judging by bed slopes flanking the lake, the water depth is at least 10s of metres. Calculations of basal temperature reveal the ice base to be warm both now and during full glacial periods. As the environments of subglacial lakes are broadly similar, life may be expected in Lake Ellsworth as in any other. Given this, its physical characteristics, and the fact that the West Antarctic Ice Sheet has been accessed on several occasions, Lake Ellsworth is an excellent candidate for in situ examination.