The depth at which mankind can work in the ocean is about to double.

In a giant step downward, toward the seas' oil, minerals and other treasures, a Duke University scientist has learned to increase human divers' practical working level from today's common 1,000-foot maximum to 2,132 feet, or four-tenths of a mile.

As proof, three happy men expect to emerge from a thick-walled high-pressure chamber at Duke Medical Center here today after spending 27 days making the world's deepest simulated dive.

The key to the achievement is a new mixture of breathable gases devised by Dr. Peter Bennett. It consists of oxygen, helium and -- surprisingly -- nitrogen.

Perversely, either nitrogen or helium can form bubbles in the bloodstream, causing the dreaded bends. As cautiously used here, however, the nitrogen in the divers' air acts as a mild narcotic.

And it counteracts the devastating effects of high pressure, which otherwise sickens divers at great depths and makes them unable to work.

Bennett's discovery is expected to lead quickly to ocean exploration at unprecedented depths, probably well below 2,000 feet.

H. G. Delaize, president of Comex, a noted French diving firm has wired Bennett and his divers congratulations on their "step toward the riches of the underwater world."

Six years ago, a Comex crew "dove" to 2,000 feet in a pressure chamber in Marseilles. But the divers were so sick -- tired, nauseated their hands shaking -- they could do little effective work.

Divers' steady hands, not just machines or the mechanical arms of small submarines, are needed for much fine work in installing or maintaining offshore oil rigs or doing other undersea tasks.

In the world of ocean exploration, studies in pressurized tanks have steadily lowered working depth.

A British anesthesiologist, Bennett came to Duke in 1972. He directs a combined deep-sea, high-altitude research center, with a series of tanks also used for medical treatments. For example, many patients with gas gangrene can be treated successfully only in high-pressure chambers.

Gradually, Bennett learned that it was the pressure on the brain of tons of ocean water than caused deep divers' shaking and nausea. He named the condition High Pressure Nervous Syndrome.

Last December, a U.S. Navy team made a research dive in a land chamber in Panama. At just 1,500 feet they suffered such severe Nervous Syndrome and weight loss that some are still recovering.

The membranes of these divers' brain cells had simply become too tightly packed. Adding just the right amount of nitrogen, Bennett found, prevents this.

Much work with humans and animals lead to the formula for the current dive: 10 percent of nitrogen in the oxygen-helium mixture deep-divers breathe.

On March 6, the Duke volunteers -- commercial diver Stephen Porter, 24; medical student William Bell, 25, and physician's assistant Delmar (Bud) Shelton, 40 -- entered the two-room diving chamber, an eight-foot ball atop a six-foot-wide cylinder.

Not until March 12 was the air pressure increased enough to "lower" them to 1,841 feet. Then, stage by stage, they were lowered to 2,004, then 2,132 feet -- 650 meters.

They stayed at the record depth for 24 hours, doing mechanical and mental tasks and reporting that they felt "fine." Then on March 15 the gradual decompression began to get them back to the "surface" safely.

Only one diver, the 40-year-old Shelton, felt any untoward effect at the maximum depth, a feeling for a time that he couldn't breathe enough air. But he worked well, and lung function tests showed he was getting ample air.

"I think he's gotten just a little old for this work," said Bennett, who is 48.

Old or not, diver Shelton looked healthy last week as he moved around a decompression tank, larger than the deep-diving chambers.

For a brief period Saturday, two of the divers felt some leg pain. Bennett recognized this as a signal that the bends were beginning to develop and temporarily returned the divers to a slightly deeper level. This is a common event as divers are decompressed, and further delays were possible.

Late yesterday the divers were practically "at the surface" and due to exit at 8:53 a.m. today, unless untoward symptoms appeared.

In real-life deep dives, divers are often lowered in small tanks so they can gradually get used to greater pressures. The tanks are capable of maintaining the pressure so the divers may be brought up and decompressed in a tank aboard ship over many days.

In real dives in the future, Bennett predicted, "I think you could get down to 1,500 feet in 12 hours. I think you could spend two or three days at around 2,000 feet. Then I think you could get out of a decompression chamber after seven or eight days."

All this, said a telegram from Dr. Jeff Davis, president of the Underseas Medical Society, "is comparable to the landing of the first astronauts on the moon."

"Your efforts," wired Ocean Engineering of Houston, "will open the door to vast areas of the ocean which were previously unavailable."