We present newly acquired airborne radar data showing ice thickness and surface elevation for Pine Island Glacier, Antarctica. These data, when combined with earlier measurements, suggest the presence of a lightly grounded area immediately above the grounding line of Pine Island Glacier. We identify this region as an “ice plain”. It lies close to the centre line of the glacier, has an elevation above buoyancy of <50 m and extends inland for >28 km. The upstream edge of the ice plain is defined by a “coupling line”. The configuration of the ice plain implies that nearby thinning of the ice stream would result in substantial grounding-line retreat. We suggest that the grounding-line retreat of Pine Island Glacier, observed between 1992 and 1996, probably commenced sometime after 1981.