Trimix family : Hypoxic Trimix
Less popular than Normoxic trimix and Hyperoxic trimix in the Trimix family, the hypoxic trimix is oxygen-depleted.
This one is concentrated at an oxygen content lower than 18%, then we adjust the balance between helium and nitrogen in order to achieve a specific dive.
Hypoxic trimix isn’t breathable in surface. This means that the diver need to have a specific tank with this mix in addition to his normal mix. (Or to dive with a rebreather that will create gas mixes automatically, but this is another story). It is used to go deeper than -60 m.
Here’s how divers are switching gas underwater: