The majority of confirmed impact craters on Earth have been discovered on land. However, the increased use of high-resolution, full-bottom-coverage seafloor mapping methods such as multibeam bathymetry have enabled the discovery of previously unknown subaqueous impact craters on continental margins and in large lakes. High-resolution bathymetric and sub-bottom profiler data collected south of the city of Sept-Îles in the northwestern Gulf of St Lawrence, eastern Canada (Fig. 1a), revealed a previously unreported circular structure of possible impact origin located 20 km off the mainland in water depths of 40–208 m (Fig. 1b, c) that had been overrun by the Laurentide Ice Sheet (Lajeunesse et al. 2013).