A large truncation surface has been discovered on seismic reflection profiles along the distally steepened ramp of the west Florida carbonate platform margin. This structure, interpreted as a submarine slide scar, is at least 120 km across and up to 30 km wide; the slide has removed as much as 300–350 m of late Paleogene and early Neogene strata. Collapse occurred in the middle Miocene within an episode of seaward progradation during a relative sea-level highstand. Postcollapse depositional sequences thicken abruptly across the structure; this suggests catastrophic slope failure. Gravitational instability, created by high rates of sediment accumulation, is believed to have been the triggering mechanism. Such large-scale platform-margin collapse is likely to generate megabreccia debris flows as well as aid in the landward retreat of precipitous, high-relief carbonate escarpments.