Abstract: A mixed-gas, closed circuit, re-breather diving unit (11), and associated diving system and method, having separate supplies (21, 22) of oxygen and inert gas, arranged to feed a breathing loop (23, 25, 26) in such a manner that, in use, oxygen is fed into the loop (23, 25, 26) as it is consumed by a diver, while inert gas is fed into the loop (23, 25, 26) by a water-pressure-sensitive valve (37) to maintain the loop volume, the unit (11) having oxygen partial pressure (ppO.sub.2) monitoring sensors (41) whereby the ppO.sub.2 is continuously monitored and maintained at a pre-set level below that of the oxygen toxicity threshold, having a depth gauge diaphragm (43) and having carbon dioxide (CO.sub.2) scrubber means (23) within the breathing loop (23, 25, 26), and the pre-set level of the ppO.sub.2 can, once the ppO.sub.2 monitor has been calibrated initially, be reset instantly to another ppO.sub.