Hypothesized drawdown of the East Antarctic Ice Sheet (EAIS) through the ‘bottleneck’ zone between East and West Antarctica would have significant impacts for a large proportion of the Antarctic Ice Sheet. Earth observation satellite orbits and a sparseness of radio-echo sounding (RES) data have restricted investigations of basal boundary controls on ice flow in this region until now. New airborne RES surveys reveal complex topography of high relief beneath the southernmost Weddell/Ross ice divide, with three subglacial troughs connecting interior Antarctica to the Foundation and Patuxent Ice Streams and Siple Coast ice streams. These troughs route enhanced ice flow through the interior of Antarctica but limit potential drawdown of the EAIS through the bottleneck zone. In a thinning or retreating scenario, these topographically-controlled corridors of enhanced flow could however drive ice divide migration, and increase mass discharge from interior West Antarctica to the Southern Ocean.