The subglacial, geothermal lake beneath the Western Skaftá cauldron (depression) in the Vatnajökull ice cap, Iceland, was accessed by hot water drilling through the overlying 300 m-thick ice shelf. Most of the ca. 100-m water column was near 4.7°C, but was underlain by a distinct ∼10 m-deep water mass at 3.5°C. The sensible heat content of the lake water is approximately twice the potential energy dissipated in outburst floods, and the temperature of the lake may be an important factor in the development of subglacial water courses of jökulhlaups from the lake. The lake temperature is higher than the temperature of maximum density, implying that convective heat transfer can take place in the lake. The vertical temperature structure suggests a large-scale recirculating flow in the lake, the rate of which was estimated from the lake temperatures and the chemical composition of a water sample.