Sea Shadow (IX-529) was a test craft developed under a combined program by the Defense Advanced Research Projects Agency (DARPA), the U.S. Navy, and Lockheed Martin Missiles and Space Company (LMSC). Its purpose was to explore a variety of new technologies for military surface ships.
To keep Sea Shadow out of the public eye, she was built inside HMB-1. Night tests were conducted in 1985 and '86 off the Santa Cruz Islands in Southern California with the barge keeping the ship under cover for repairs and replenishment during daylight
The Hughes Mining Barge 1 (HMB-1), is a uniquely designed submersible barge. She was originally built in 1973 as part of Project Azorian, the top-secret effort by the Central Intelligence Agency to salvage the remains of Soviet submarine K-129 from the ocean floor. HMB-1 was designed to be submerged under the Glomar Explorer to secretely load the large claw device that grappled the submarine, and to conceal the recovered submarine.