This diver purchased an old Soviet rebreather (scuba system that reuses air rather than just rely on a big tank) on eBay and shockingly was able to make it work. This is a fascinating project because of how dangerous and difficult such a project is.
"IDA-59 is a Russian fully closed circuit[see 2.1] trimix rebreather. The original purpose of the device is to be a submarine bail-out apparatus. Maximum operation depth is 300m (1000ft) according to the original manual. This requires an additional diluent cylinder, which is connected to the off-board diluent connector. Without this extra dil cylinder, the maximum operation depth is limited to about 90 m (300ft).
IDA is an acronym, which stands for Russian words (in non-cyrillic letters) individual'nyy dykhatel'nyy apparat (meaning "individual breathing apparatus" in English)."
IDA - 59 Trimix Rebreather Project