Hiawatha Crater in northwest Greenland is one of only two confirmed impact craters under an ice sheet. Hiawatha therefore offers a rare proxy for understanding the interactions between active glacial dynamics and impact craters on other planetary bodies. Here we characterize Hiawatha’s subglacial environment, presenting the results of two active-source seismic experiments and interpreting them in concert with previous radar sounding analysis. Seismic reflectivities at a site with a single basal radar reflector are consistent with a substrate of impact melt-bearing breccia. At a site where radar sounding revealed a porewater reflector 10–15 m below the base of the ice sheet, our seismic results are consistent with a substrate of unlithified sediments. We propose a model where supraglacial water supplies heat for basal melting, leading to rain–out of unconsolidated sediment from dirty basal ice (fringe) and permitting a permeable porewater-bearing basal layer to persist without consolidating.