I don't recommend anyone "inject" trimix ;) But jokes aside : Trimix is an (artificial) gas blend that includes helium, which is an inert gas that your body doesn't metabolize (though their is still a lot of unanswered questions about its effects below 250 meters). Your body metabolizes gases (normally only Oxygen) according to their partial pressures in your body. The partial pressure of Oxygen at sea level is just 21% (there's an atmospheric pressure of 1 Bar and air is 21% Oxygen, 78% Nitrogen, and 1% Aragon and other weird stuff), but as you dive deeper into the water and the ambient pressure rises, so does the partial pressure of Oxygen and Nitrogen. Oxygen becomes toxic around (to be safe) 1.2 ppO2 and Nitrogen causes narcosis at a pp that varies greatly from diver to diver. As a rule of thumb, somewhere between 40 and 60 meters is where things get dangerous. So, to reduce the partial pressure of these gases, Helium is introduced, reducing the proportion of the metabolized gases in the total mix, and allowing the diver to go deeper before the critical thresholds are reached. To dive below a certain depth (about 60 meters), a mix is required where the proportion of Oxygen is less than the minimum 17% Oxygen at 1 Bar required for your body to function. We call these mixes "Hypoxic Trimix". A typical deep technical dive involves switching gases (different tanks) at least three times during the dive, and grabbing the wrong regulator is a deadly mistake. Technical diving is not for the distracted. It is, also, the slowest and most meditative rush you'll ever experience. Add overheads and caves and it really does feel like an excursion to an other planet. Speaking of space travel : more humans have been to space than below 200 meters.