Trimix is a mixture of nitrogen, oxygen, and helium used in deep technical diving. Helium replaces part of the other two found in normal air, lessening the adverse effects they have at depth: narcosis, decompression sickness, oxygen toxicity. The purpose is to facilitate reaching depths of 160' and well beyond.
A typical trimix dive goes something like this: the diver breathes from a tank of normal air while decedning to a depth where he can switch to the trimix, or "back gas" since that is what he carries on his back and breathes durring the deepest part of the dive.
On ascending he might go back to the air bottle until he reaches a nitrox (oxygen-enriched air) tank he has attached to the mooring line on the way down. This is called "deco gas" and is breathed for decompression and surfacing.
There are all sorts of variations on this process depending on dive profile, availity of helium, and lots of other stuff. It requires a good deal of advanced training and gear, and can get quite expensive.
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What is Tri-Mix 
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