With increasing depth, the nitrogen in air (or nitrox) becomes increasingly narcotic and increasingly dense and difficult to breathe. Therefore, helium-oxygen or helium-nitrogen-oxygen breathing mixtures are used for deep dives, because helium is not narcotic and less dense than nitrogen. Aside from these clear benefits of helium, other differences are less well understood. It is thought that faster uptake of helium than nitrogen into the body during bounce dives results in a greater decompression obligation. Switching to air or nitrox during ascent is thought to accelerate decompression but also to risk inner-ear decompression sickness. If DCS occurs, the typical symptoms following helium-based diving are thought to be different from nitrox diving. Is any of this true?