Results of continuous high-frequency seismic reflection profiler and towed magnetometer surveys of Mary Bay are presented. These data were taken, along with several piston cores and heat flow determinations, as part of a broader reconnaissance survey of Yellowstone Lake. The bathymetric data reveal an oval basin occupying the center of the bay, bounded by steep slopes. The southeastern slope appears to trend onshore, where steep bare scarps facing the lake are exposed near the north and east sides of the bay. The steep slope surrounding the basin in the bay corresponds to a change in character of the magnetic field. Broad smooth anomalies are found over the basin, and narrow high-amplitude anomalies over the surrounding platform. The center of the basin is occupied by deep narrow thermal basins having extremely rough local topography, where very high values of heat flow and sediment temperatures were measured. These data support the hypothesis that Mary Bay is the site of a submerged hydrothermal explosion crater similar to others onshore.