From Wikipedia:

USNS Glomar Explorer (T-AG-193) is a large ship currently being used as a deep-sea drilling platform. The vessel was built for a secret operation, Project Jennifer, by the United States Central Intelligence Agency to recover a sunken Soviet submarine, K-129, which had been lost in April 1968.

Since the K-129 had sunk in very deep water, a large ship was required for the recovery operation. However, such a vessel would easily be spotted by Soviet vessels, who might interfere with the operation and so an elaborate cover story was developed. The CIA contacted the businessman Howard Hughes, who agreed to assist.

Click on the link and read the Project Jennifer entry too. Fascinating stuff.