Underwater diving uses self-contained underwater breathing apparatuses (SCUBA) that allow divers to breathe underwater independently of surface supply. SCUBA systems include high-pressure diving cylinders that hold compressed air or gas mixtures, and one- or two-stage regulators that reduce cylinder pressure and deliver breathing gas on demand to the diver. While early regulators were single-stage, modern two-stage regulators have become popular as they deliver air more reliably during inhalation.