The hypothesis that an impact crater underlies Hiawatha Glacier in northwest Greenland was motivated by serendipitous NASA airborne radar sounding, but geologic field mapping and a focused survey using a new ultrawideband (UWB; 150&ndasg 520 MHz) radar system onboard the Alfred Wegener Institute’s Polar 6 Basler DC-3T aircraft were essential to confirm this hypothesis. Here we describe the multiple anomalous subsurface features observed by this survey and discuss the broader potential value of UWB airborne radar sounding of ice sheets. In the near-surface of Hiawatha Glacier’s ablation zone, discrete cross-bedded units up to tens of meters thick are likely superimposed ice associated with advected former supraglacial lakes. Elsewhere, numerous emerging reflections intersect the surface where satellite-observed outcrops of units with known visual characteristics from the Holocene epoch and the Last Glacial Period (LGP), and radar-reflectivity and surface-stratigraphy patterns are self-consistent. The Holocene–LGP interface is conforming above the northeast half of the crater but undulates dramatically at sub-kilometer scales above its southwestern half, likely recording a significant change in ice flow of unknown origin. Within the basal ice above the crater, numerous bed-originating reflectors emanate mostly from the central uplift, and point scatterers are common, which we interpret as evidence of basal freeze-on over a bedrock obstacle and vigorous subglacial erosion of the crater, including quarrying of boulder-sized clasts. Finally, a remarkably flat reflection typically 15 m below the ice–bed interface is sometimes detected at the downstream end of the crater, which likely represents the first detection of a subglacial groundwater table below presumably well drained impact breccia. As with previous generational advances in airborne ice-penetrating systems, we should expect that UWB surveys of ice sheets will detect previously unobserved subsurface features and identify new directions in radioglaciology. While some of the reported features may be unique to Hiawatha Glacier, our observations demonstrate the value of this new UWB airborne radar system to advance our understanding of processes at ice-sheet margins, which have historically been a challenging radioglaciological target.