Worth reading the linked page [0] about one of the diver's custom reassembled rebeather, which means they have two systems to reduce the need for backup tanks of air, but still be able to fit in the caves. Really incredible, particularly as they work as volunteers.
I can recommend the documentary film "Diving into the unknown". About Finnish cave divers that dive to return the bodies of their dead friends in Norwegian cave.
http://www.divingintotheunknown.com/en
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