Recreation: Cryogenic Scuba

The big limitation for scuba divers is not how deep they can go but how long they can stay under. Conventional compressed-air breathing units, which weigh 36 Ibs., are generally exhausted after one hour. Soon, according to this month's Skin Diver magazine, aquanauts will be able to submerge for six or even eight hours at a time with a back pack that weighs only half as much as usual.

The secret is liquid air—a mixture of oxygen and nitrogen supercooled to — 318°F. It is pumped into vacuum-insulated Dewar tanks, sophisticated thermos bottles that protect the icy liquid from the warmth of the surrounding water and at the same time keep the diver's back and shoulders from freezing. From the tanks, the liquid air is piped through warming coils that heat it until it expands into breathable gas. Only hazard: since liquid air allows the diver to stay under far longer, he must surface slowly, in stages, to avoid the bends.

The breathing system is called cryogenic scuba, for the science of supercooling, which has been used to fuel spacecraft with liquid oxygen and, in medicine, to freeze everything from ulcers and tumors to tonsils and cataracts. The new scuba rig was pioneered by Jim Woodberry, 23, a Miami diver who has successfully tested a prototype for a total of 400 hours at depths up to 200 ft. He plans to have it on the market before year's end. Anticipated price: $250 to $300 for the apparatus, plus $3.50 for each refill of liquid air.