USS Dolphin in downtown San Diego holds the world record as the deepest-diving operational submarine, reaching depths exceeding 3,000 feet after her 1968 commissioning as AGSS-555 at Portsmouth Navy Yard. The vessel is part of the Maritime Museum of San Diego's floating collection along North Harbor Drive on the Embarcadero near the USS Midway Museum. Designed as a diesel-electric research and development platform rather than a combat vessel, Dolphin's HY-80 steel pressure hull used deep frames instead of bulkheads, allowing internal modifications for Navy and civilian experiments over a 38-year career. That career produced a series of firsts: the deepest torpedo launch ever recorded in 1969, the first submarine-to-aircraft optical communication, and early testing of what became the fleet's BQS-15 sonar system. Visitors walk through the 151-foot hull alongside the museum's other historic vessels, including the replica tall ship HMS Surprise. The submarine's final configuration supported deep-water acoustic research, oceanic surveys, and sensor evaluations that shaped modern undersea warfare technology.