It is a diving technology that actually predates SCUBA by hundreds of years. SCUBA is "open circuit": a breath is taken from the gas in your tank and then expelled.
Rebreather technology is "closed circuit": air is reused by scrubbing carbon dioxide from it with a filter, oxygen replenished from a cylinder, plus other "filler" gas is added. The advantages of this include more bottom time since gas is recycled rather than expelled, the gear is less cumbersome, very deep dives become more practical, and it is almost silent.
The disadvantages include the complexity of using, prepping, and maintaining the gear, the extensive training required, the low tolerence for errors, and the expense.
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I am not the one who needs mental help. I just need to vent. | 
What is a rebreather 
Linear Mode
