Inventors have long sought an underwater breathing apparatus that doesn’t store oxygen, but extracts it from the water the way gills do. Israeli inventor Alon Bodner has come close.
The device, aptly named LikeAFish, works by using a centrifuge to lower the pressure of water within an airtight chamber. Since only a little oxygen is contained in water, the device must move about 190 liters (50 gallons) per minute in order for the average person to breathe comfortably. Despite this, the only real barrier to implementation is size and weight, but it’s close enough that the device has been under consideration for military use for several years now.
Such a system would obviously allow for longer “bottom time” without the need for refilling oxygen and would decrease the amount of nitrogen the diver is exposed to. According to Bodner’s website, the company spent 2012 “quietly designing a prototype to be installed on board a naval submarine,” so they may be very close to solving the size and weight issues of previous prototypes.