The first impact crater in the ocean has been identified on the outer continental shelf off Nova Scotia, Canada. The crater is well preserved and buried by 510 m of Tertiary and Quaternary marine sediments. The crater is a circular structure at least 45 km wide and 2.7 km deep, with a central structural uplift. The uplift is at least 1,250 m and possibly 2,900 m in height. Its upper surface is 11.5 km across, and it has a central pit 3.5 km wide in the center. An exploratory oil well located near the center of the structure encountered Cambro-Ordovician metamorphic rocks of the "basement," which are fractured with planar deformation features in quartz representative of pressures in the 8- to 12-GPa range. The uplifted basement is covered by 552 m of breccia. Shock-induced features are common; they include planar lamellae in quartz and feldspars; partial to total isotropization of silicates; and the occurrence of mixed tectosilicate glasses, some of which exhibit flow textures with total dissociation of the mafic components to ill-defined aggregates. These features are diagnostic of a 35-to 50-GPa shock level for parts of the breccia. The top of the breccia is covered by a 40.5-m-thick suevite zone. Two crystalline melt-rock horizons 72 and 34 m thick are enclosed in the breccia. Even though the feldspars in the melt are labradorite to bytownite in composition, the chemical composition of the rock is rhyolitic, with 70%-77% of SiO2, similar to the composition of the basement. There is no obvious enrichment in siderophile elements, except an increase in iridium (0.1-0.3 ppb). The occurrence of a thick section of impact melt rocks and breccia capping the central uplift is a feature not recognized in other complex impact craters and may be a function of impact into a marine environment.Dating by 39Ar/40Ar indicates that the impact occurred at about 51 Ma, late early Eocene, and this is confirmed by biostratigraphic data from immediately overlying sediments. The compositional and structural imprint left on the Montagnais structure by the colliding body points to the impactor being most probably the nucleus of a comet.