Director James Cameron successfully completed a 6.8-mile-deep dive to the most remote region of the ocean Sunday and was shown emerging from his submarine in a small knit cap. Jacques Cousteau’s red knit cap was a signature part of his look, which was aped by Bill Murray and his crew in the movie The Life Aquatic With Steve Zissou. Why do underwater explorers wear skullcaps?

Because it’s practically freezing down there. The water temperature at the bottom of the ocean usually hovers around 37 degrees Fahrenheit, and most deep-sea exploration vehicles don’t have climate control. Explorers tend to bring hats, gloves, long johns, and other warm layers, which they pull on as they descend and the temperature drops. Socks and caps are particularly important, as it’s coldest on the floor and ceiling of the submersible. Because of concerns over electrical fires, deep-sea explorers wear natural fibers like cotton and especially wool, which is fire retardant, instead of synthetic fabrics.

Deep-sea divers have been wearing skullcaps, also known as watch caps or seaman’s caps, since long before the adventures of Jacques Cousteau. He may have picked up the style from hard-hat divers—those 19th-century explorers who wore big copper helmets—who favored red knit caps for decades. The character of Steve Zissou in The Life Aquatic wore an identical cap in homage to Cousteau.

Aside from the extreme temperature, diving in a deep-sea submersible is much like flying in an airplane, except a much tighter squeeze. In order to withstand the crushing pressure of deep waters, the vehicles must be compact and often have spherical cabins. Changing clothes is difficult because there’s rarely enough room to stand. Going to the bathroom, for which there is little privacy, is a particular challenge for female explorers, who have to contort themselves more than men to use the urine-collection bottles. (The bottles come with different attachments for men and women.) While divers may avoid drinking fluids in order to put off urination, they must also be careful not to become dehydrated. For long dives, air can be recycled using carbon dioxide scrubbers and oxygen generators. (Scientists have attempted to develop liquid breathing, the technology used in Cameron’s deep-sea thriller The Abyss. Just as in the movie, mice were able to adjust to breathing the liquid. However, the application of liquid breathing for human diving remains hypothetical.) Still, these technologies can’t always keep up with passengers’ demand for oxygen, and enough carbon dioxide can accumulate to cause headaches. The cabin is pressurized enough that divers will feel little of the extreme pressure outside of the vehicle—dangers like nitrogen narcosis and the bends aren’t a concern in a functioning vehicle—but their ears might pop when the submarine is opened and repressurizes at the surface.

The prospect of deep diving may seem exciting and include moments of thrilling discoveries, but the trip itself is hardly a thrill ride. Vehicles usually travel at a maximum speed of only 2 or 3 miles per hour, so divers sense little of the motion of ascent and descent. Since it can take several hours to reach the seabed, most of the trip can be spent just passing time. Pilots and solo explorers like Cameron must be mindful throughout the journey, but passengers may simply gaze out the porthole (though the water goes dark after about 10 minutes), read, or doze off; some occupy themselves with a computer or music. When resurfacing, however, turbulent waters around the surface can feel like an amusement park ride, or worse, and many craft don’t have seats or seat belts.

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Explainer thanks Barbara Campbell and Craig Cary of the University of Delaware.

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