You get pretty cold pretty fast when you’re wet. Water absorbs more heat than air—and absorbs it 20 times faster. Without some kind of protection, people can suffer hypothermia even in warm tropical seas.

There are several ways to stay warm in the deep blue, starting with wetsuits and drysuits and adding piped-in hot water or portable heating systems.

Surprisingly, the U.S. Navy is still looking for a solution to the cold-water problem for its divers. Its nuclear wetsuit didn’t quite work out.

For military divers, water’s chill factor complicates the challenges they already face. Cold affects energy levels, dexterity and thinking. Even simple tasks like turning a wrench or opening a hatch become draining in cold water.

Add extreme depths and the need for stealth and the complications become dangerous indeed.

Most divers today use neoprene wetsuits to keep warm. Neoprene is essentially a kind of very tough thick foam rubber. A wetsuit not only insulates a diver from the cold outside water, but also traps a layer of water next to the skin, where the diver’s body heats it.

Old-school hard-hat divers kept warm wearing woolen longjohns and knit caps inside their waterproof canvas suits and heavy copper helmets.

Modern drysuit divers use a similar system, wearing undergarments akin to outdoorsmen’s long underwear beneath their drysuits.

The deeper you go, the colder it gets—and the gas mixtures required for deep dives suck the heat from a human body. The helium in deep-diving gas mixtures conducts heat much more efficiently than ordinary air does. Beyond certain depths neoprene gets squeezed too much to insulate effectively. A drysuit or underwear isn’t enough to make up the heat loss.

Commercial and military divers going really deep most often use umbilicals that supply their breathing gas. These umbilicals include hoses that pump heated water into special undergarments to fend off the chill.

Umbilicals run to a surface vessel or a diving bell supplied from a ship. While essential for long deep-sea dives, they’re not always practical.

Keeping deep divers warm has challenged the Navy for almost half a century … and it’s still working on the problem.