When you add helium to your tank on a dive, the resulting mix is usually called "Trimix".
When you get to technical dives requiring Trimix, one often dives with several different gasses. The gas you breathe at the deepest point of the dive would be toxic if you were to breathe it at a shallower depth.
When you get to technical dives requiring Trimix, one often dives with several different gasses. The gas you breathe at the deepest point of the dive would be toxic if you were to breathe it at a shallower depth.