Hollis Prism 2 RebreatherTechnically Focused The PRISM 2 is an electronically or manually controlled, constant PO2, modular, closed circuit diving system. As open circuit (SCUBA) divers, we rely upon filling our lungs with air from a pressurized gas tank with every inhalation and then exhausting that volume of air into the surrounding water with every exhalation. As we descend deeper, the water pressure around us increases, and the volume required to fill our lungs increases;