DSV-4 (ex-Sea Cliff) is a 25-ton, manned deep-ocean research submersible owned by the United States Navy. It is sister to DSV-3 Turtle, Alvin (DSV-2), and also an Alvin class Deep Submergence Vehicle. The Sea Cliff was retired from active service in 1998. Per the Naval Vessel Register [1], DSV-4 was returned to active service on September 30, 2002. It is known only by its hull number, not by name.
The DSV-4 originally had a maximum dive depth of 6500 feet (2000 m), like all Alvin-class DSVs at first. It was redesigned to dive to 20,000 feet, and refitted in 1984. With the refit of DSV-4, DSV-1 (ex-Trieste II) was retired from service. It has a hatch 24 inches in diameter, and held in place mechanically with hatch dogs and while submerged by the pressure of the water above it (it is tapered, narrower inward). The Alvin-class DSVs were designed to replace older DSVs, such as the less maneuverable Trieste-class bathyscaphes. The DSV-4 can dive 5,000 feet further than the famed Alvin, however the super Alvin class replacement for DSV-2 is designed to dive to 22,000 feet.