Small ocean basins, such as the Aleutian basin, Gulf of Mexico, and Black Sea, are underlain by crustal sections that are significantly different from normal continents or ocean basins. A common layer with oceanic crustal velocity has a wide range of thicknesses. A very thin layer with granitic velocity occurs in some places. Both are absent under a few basins where sediment rests on low-velocity mantle. These basins are the locus of intense sedimentation, and most are floored by abyssal plains underlain by many kilometers of sediment. The basins have only a small total area, but they contain about a sixth of all identifiable oceanic sediment. In some basins the thick sediment has been pierced by salt domes or possibly giant mud lumps. A common type of small basin has a normal oceanic layer overlain by thick sediment and possibly underlain by high-density mantle. Sediment in these basins will ultimately approach continental thickness unless deposition is interrupted. The basins have about 5% of the area of continents. They may mark regions of crustal transition from typical oceanic to typical continental crust. If so, such transitions may not be uncommon. Transitions from continental to oceanic crust are less evident.