The Orca Basin, on the northern Gulf of Mexico continental slope, is unique among the many intraslope basins of the area, in that it contains a large brine lake at the base of the water column. The brine lake is of scientific significance on many levels: as an unusual hypersaline marine habitat; a laboratory for study of the preservation of organic and inorganic matter in anoxic environments; an anomalous geophysical phenomenon; and a closed system where the dissolution of rock salt in the submarine environment can be investigated. The latter two subjects are the focus of this article. The brine lake occupies an area of approximately 123 km(2) (47.5 mi(2)) in the base of the Orca Basin and is up to 220 m (721 ft) deep in places. High-resolution multifold three-dimensional seismic reflection data enable this unique feature to be mapped and described quantitatively for the first time. An updated physiographic description of the Orca Basin and a calculation of the volume of the brine lake is presented. At 13.3 km(3) (3.2 mi(3)), the volume of brine represents the dissolution of about 3.62 billion t of Jurassic Louann Salt in seawater. We calculate the time-averaged addition of salt to the Orca Basin through the dissolution of exposed salt at nearly 0.5 million t/yr.