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On 11th April 1968, the Russian Golf II class submarine K129 was lost some 600 miles northwest of Hawaii. Despite an intensive search, the Russians were unable to find the wreck. But using the SOSUS underwater sound detection network, the United States Navy was able to locate the area where the submarine was.
The Golf II was not the most modern of Russian submarines, but she was carrying 3 SS-N-5 nuclear missiles, along with sonar and radar equipment. Perhaps most importantly, she would also have been carrying codebooks and encoding equipment. If these could be salvaged, NATO forces could decode Russian messages easily.
But the USN did not know the exact location of the wreck, nor if it was intact. So the special operations submarine SSN587 Halibut was sent to the area, along with the surface research ship AGOR11 Mizar. The wreck was soon pinpointed, but it lay in some 16500 feet of water, more than 3 miles down.
The salvage operation was headed by the CIA, with cooperation from the USN. As there was no submarine capable of operating at those depths, a specialist salvage ship was required. Reclusive billionaire Howard Hughes paid for the Glomar Explorer to be built, pretending that the 63000 ton ship was for deep sea mining. The Glomar Explorer would not lift the submarine on her own, however.
HMB1, or Hughes Mining Barge 1, was a submersible barge with a large claw underneath it. The claw was intended to grab the wreck, and then a hoist in the Glomar Explorer would raise both barge and submarine up into a moonpool inside the ship.
Extensive preparations were made, and it was not until 11th August 1974 that President Ford gave his permission to begin the operation, when the Glomar Explorer was already in position over the wreck. The operation was carried out over the next month.
When the claw had grappled the wreck, the lifting process began. Slowly the submarine was raised from the seabed, and when the Glomar Explorer was carrying the entire weight of the submarine, some 3000 tons, she was lying 7 feet lower in the water. The operation went well until the wreck was some 5000 feet from the surface. Then part of the claw broke off, and the wreck started to disintegrate, two of the nuclear missiles tumbling to the seabed below.
The exact details of what was recovered are still secret, but officially only the forward 38 feet of the submarine was raised, including the bodies of 8 Russian sailors, a pair of nuclear tipped torpedoes and several encoding devices. The bodies of the Russians were buried at sea in a bizarre ceremony which was filmed and recently released to the public.
Exactly what was recovered remains a source for debate. Some say that the entire wreck was raised, but it seems that the submarine would have been too large to fit inside the moonpool, even assuming that the story of the claw breaking is false.
Despite the apparant success of the mission, the public had no idea what was going on. But four burglars broke into Hughes office and stole thousands of dollars in money, as well as files about Project Jennifer, scattering some of them as they made their escape. Thinking the files contained business information, the thieves demanded a million dollars for their return. The FBI and Los Angeles police were brought in to recover the documents.
Most were recovered, but the tight secrecy placed on the theft was not good enough. The details were leaked to the media, and in February 1975 the Los Angeles Times broke the story of Project Jennifer to the world.

The Hughes Glomar Explorer's Mission
From Andrew Toppan's sci.military.naval FAQ:
Project Jennifer was the codename applied to the CIA project that salvaged part of a sunken Soviet submarine in 1974. The Soviet Golf-class ballistic missile submarine (SSB) K-129 sank off Hawaii on 11 April 1968, probably due to a missile malfunction. The Golf class submarines were diesel-electric ballistic missile subs, a modified version of the Foxtrot class submarines. They carried 3 SS-N-5 SLBMs in an elongated sail structure.
The sunken submarine was located in 16,500 feet of water. Mizar (AGOR 11) took part in the search, as did the specialized "research" submarine USS Halibut (SSN 587). It is possible that the "research" sub Seawolf (SSN 575) also took part in the search (see section F.8 for information on US covert operations submarines).
The CIA ran an operation to recover the sunken submarine. The recovery effort centered on Hughes Glomar Explorer, a 63,000 ton deep-sea salvage vessel built for the project. The ship was built under the "cover story" that she was a deep-sea mining ship, intended to recover "manganese nodules" from the ocean floor. The ship was supposedly being built for the Summa Corporation at the direction of Howard Hughes for use by his Global Marine Development Inc. At the same time the "Hughes Mining Barge" was built. The barge, commonly known as HMB-1, was a submersible barge intended to carry the "claw" to be used in the recovery effort; it would also be used to hide the recovered submarine.
Hughes Glomar Explorer was equipped with a massive hoisting mechanism amidships and a "moon pool", a large internal underwater hangar to provide access to the ocean. The submarine was to be hoisted by a massive claw, which was stored in HMB-1. After Hughes Glomar Explorer and HMB-1 left port, the barge submerged, manuvered under Glomar Explorer, and the claw was hoisted into the moon pool. Glomar Explorer arrived on the recovery site 4 July 1974 and conducted salvage operations for the next month. If the entire submarine had been recovered it would have been stored in HMB-1 after the salvage. In the event (according to the story released to the public), only the forward 38 feet of the submarine was recovered. The section included two nuclear-tipped torpedoes, various cipher/code equipment and 8 dead crewmen. The recovered section was small enough to be brought into the moon pool, where it was analysed and disected. The dead Soviet sailors were buried at sea. It is possible that the entire submarine was recovered and the story about only the bow being recovered was further "cover".
After the recovery Hughes Glomar Explorer was transferred to the Navy on 3 Sept 1976 and designated AG 193. The vessel is not officially assigned a name, but is commonly referred to Glomar Explorer. She was transferred to the Maritime Administration on 17 Jan 1977 and laid up at Suisun Bay, CA. The Navy attempted to sell the ship, but failed. In June, 1978 she was leased to Global Marine Development Inc. for commercial use. That lease was terminated in 1980. In 1979 it was proposed that the ship be transferred to the National Science Foundation for use as a deep-sea drilling ship, but that effort was not funded. The ship was returned to Navy custody on 25 April 1980 and transferred to the Maritime Administration on the same day for layup at Suisun Bay. She remained in layup for the next 16 years. During August, 1996 it was announced that Global Marine had leased Glomar Explorer from the Navy for 30 years. The ship left the mothball fleet 5 November 1996 to be totally reconditioned and converted to a drill ship. She will be operated in the Gulf of Mexico, drilling test oil wells.
HMB-1 was laid up after the recovery, but was transferred to the Environmental Protection Agency at some point. She was returned by the EPA in 1982, officially to be laid up in reserve. It now seems likely that it was then employed as the "mother ship" for the stealth ship Sea Shadow, a purpose for which it was employed during the 1990's. At some point HMB-1 was converted from a submersible barge with access from the top into a covered floating drydock with access from one end. It is currently in storage, with Sea Shadow inside. See E.18 for information on Sea Shadow and the test program