Abstract: Underwater breathing apparatus immediately provides two liters of fresh air from the fresh air tank to a diver at any reasonable depth as inhalation begins. The necessary additional volume of gas in order to equalize the pressure of the ambient environment is automatically derived from a mixture of fresh air and recycled air, devoid of carbon dioxide and moisture, provided by an ambient pressure compensating diaphragm and associated chamber which is also in communication with the means for connecting the apparatus with the breathing organs of the diver.