The Amazonian-aged 215 km-diameter Lyot crater represents a transient 'drill-hole' into the martian hydrosphere/cryosphere system. Cratering mechanics predict that the event should have penetrated the cryosphere and released groundwater held under artesian conditions. A plausible explanation for the lack of evidence for hydrologic activity within Lyot is an absence of abundant subsurface groundwater in the region at the time of impact.