Subglacial topography controls how Antarctic ice flows towards the ocean and responds to climate change. In the Denman–Shackleton region, deep valleys connect the ice-sheet interior to the coast. Denman Glacier occupies one of the deepest and could contribute significantly to sea-level rise. Combining new ground-based gravity measurements with airborne data and statistical modelling reveals a more complex landscape than existing maps and a potential vulnerability to unstable retreat.