The hatch would not open.

Six hundred feet below the surface of the Pacific Ocean off the Southern California coast, Chief Warrant Officer Bob Barth struggled to get inside the Navy’s new Sealab 3 habitat. The 340-ton undersea platform, where he and a nine-man crew were scheduled to spend 12 days together, was leaking, and Mr. Barth and a fellow diver, Berry L. Cannon, had headed down with two other divers to fix it.

Suddenly, Mr. Barth realized that Mr. Cannon was having convulsions, that his respirator had floated free and that his jaws were clenched shut so that it could not be reinserted.

The scene played out on the closed-circuit television monitoring the habitat from the surface; Mr. Barth frantically tried to get his crewmate back to the diving pod, where the other two divers waited. One of them, Richard Blackburn, helped wrestle Mr. Cannon back into the capsule for the slow return to the surface. As they rose, they tried to resuscitate Mr. Cannon, but without luck; using their intercom, they called to the surface to say he had died.

Mr. Cannon’s death marked the end of one of the great programs of naval exploration, one that had begun with a dry-land test called Genesis and had moved through three versions of the Sealab underwater habitat, from the late 1950s to 1969. The program greatly increased the depths at which humans could safely live and work. The divers were called aquanauts, an under-the-sea analogue to the glamorous astronauts who circled the earth and landed on the moon.