Seismic data from Rutford Ice Stream, West Antarctica show high amplitude, reversed-polarity reflections from parts of the bed. Only interfaces between ice and water, or between water and sediment have the acoustic properties to produce such strong reflections -- ice/sediment interfaces produce much weaker echoes. The evidence suggests that an area of the ice stream measuring at least 1 km by 0.2 km is underlain by water with a depth of less than 1 m. The data are consistent with the presence of a sub-ice drainage system comprising shallow canals cut into the sedimentary substrate, as predicted by theory.