Humans exhale a lot of oxygen along with their waste carbon dioxide. Instead of throwing out this oxygen, a rebreather uses a scrubber to remove the CO2 and replace it with pure oxygen from a bottle. Tom Rose built this rebreather for $100. When you exhale air passes through the scrubbing material and is stored in hot water bottle counterlung until you inhale. The system is only 15 pounds; a great savings compared to most dive equipment. You are definitely putting your life in your own hands so this should not be used without plenty of “couch-diving” tests. Tom has a ton of other diving related projects on his site.