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 Turtle (DSV-3), DSV 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 , 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 (ex-Trieste II, DSV-1) was retired from service. It has a plug hatch 24 inches in diameter, held in place mechanically with hatch dogs, and while submerged, by the pressure of the water above it. 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 deeper than the famed Alvin; however, the super Alvin class replacement for DSV-2 is designed to dive to 22,000 feet.