The air in tanks also gets cold, so divers become chillier with each breath. But in the 60s, the Navy thought they had an ingenious solution to it all: nuclear power.
Plutonium-238 is a byproduct of nuclear weapons production, but it also happens to be the ideal nuclear fuel for wetsuits—once you accept a nuclear-powered wetsuit is a good idea in the first place. It emits a lot of radiation, but only the kind that is easy to shield from. In this wetsuit design, almost a kilogram of plutonium-238 is placed inside a canister, where it radiates heat to a series of fluid-filled tubes lining the suit.
When the Navy actually tested the suit, it did not... work so well. "It is concluded that the system in its present state is incapable of maintaining thermal balance in a diver at depth, and its use under SEALAB III conditions would entail a grave risk of hypothermia".
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