The Ronne Ice Shelf, floating between Berkner Island and the Antarctic peninsula, is the biggest ice shelf in Antarctica. It drains an estimated 1.2 × 106 km2 of the Antarctic ice sheet, much of it resting on bedrock below sea level. Consequently, the balance and dynamics of this ice shelf is of importance to Antarctic glaciology, especially with regard to the integration of this part of the cryo-sphere into the global processes that control the climate of the Earth. Extensive radio-echo sounding (RES) by Robin and others revealed reflections in the central part of the Ronne Ice Shelf at the relatively shallow depth of 100–200 m below surface1. The interpretation of these echoes, which varied in strength, was ambiguous, and the possibility of internal reflecting horizons was thoroughly discussed. But after surface elevation measurements by radar altimeter from drifting balloons appeared to fit the presence of thin ice, it was decided to base a thickness map of the Ronne Ice Shelf on these RES echoes1–4. We now present direct observational evidence from boreholes that the total ice thickness is much greater than mapped, and that the shallow RES reflections therefore do come from internal horizons.