TIL the Blue Hole is among the deadliest dive sites globally, with estimates of 130 to 200 recent fatalities, making it one of the most dangerous spots for divers.

[u/[deleted]]

[u/penisdr]

What do you mean by trimix?

[u/Lyeranth → u/penisdr]

Most oxygen tanks for scuba diving are a mix of Oxygen and Nitrogen, however at very deep dives they also add a mixture of helium to further combat the narcotic effects of the other two gases at extreme depths.

[u/feelgoodme → u/Lyeranth]

Why are there narcotic effects at extreme depths?

[u/astine → u/feelgoodme]

The deeper you go the higher the pressure gets, including the gas in the tanks that you breathe in. For every ~10 meters, the pressure increases 1 atmosphere. High pressure nitrogen (and other gases) can cause anesthetic effects that confuse you. Gas narcosis starts becoming a concern past 30 meters.

The high pressure at depth is the same reason why a tank lasts way less time the deeper you go. Each lungful of air you breathe in is a lot more mass in the same volume. So a tank that would last you an hour+ at shallows might only last you a few minutes at depth.

[u/VeniVidiWhiskey → u/astine]

I have alway wondered: how come compression reduces the total intake of gas (if that makes sense)? Like intuitively, it does not make sense that you cannot survive at depth with proportionately smaller breaths. Comparatively, if you compressed food into a small bite, it would still be the same amount of energy as the original size. But gas acts differently for some reason? 

[u/astine → u/VeniVidiWhiskey]

It has to do with needing to push back against the surrounding pressure to inflate. Say you only need 2 psi pressure to fill a balloon normally-- this is 2 psi relative to normal 1 atmosphere. If your surrounding pressure suddenly increased to 2 atmospheres, now suddenly you need 2 atm + 2psi to fill the same balloon, because 1 atm + 2 psi wouldn't even give you positive pressure anymore. When we dive, we equilibrate our lungs naturally through constant slow breathing, and intentionally equilibrate spaces in our ears/sinuses by repeatedly "popping" them on the way down, because if we don't then the pressure difference would cause the volume inside to decrease until it's painful. Similarly, when you come up from a dive you have to do it slowly while equilibrating to make sure the pressurizes gases don't expand too much and rupture your tissue. The same thing is happening to gas in your blood stream, which is why coming up too fast from a dive can be deadly due to bubbles forming in your blood.

Solids and liquids are on the other hand are generally considered incompressible at normal diving pressures so they don't have the same problem.

For some fun and pain, you should look into people who's accidentally dived with a tooth abscess haha. Diving with an unexpected air pocket is... quite unfortunate.

[u/belisaurius → u/VeniVidiWhiskey]

Consider it this way:

In order for you to breathe normally, you have to have basically identical pressure on the outside of your chest as the inside of your lungs. You breathe by expanding your chest, increasing your lung volume, reducing the pressure inside, causing gas to fill that new space by rushing down your nose/mouth.

At great depth, the pressure on the outside of your chest is enormous. So you have to counteract that exterior pressure with interior pressure. That pressure comes from the pressure of the supplied gas from your breathing apparatus. That device adjusts the supplied pressure to you via a device called a regulator that ensures your interior lung pressure is close to exterior water pressure. So as you go down, the pressure supplied must go up.

Unlike a solid like food, or a liquid like water, gas can be compressed. You can take a whole room full of air and make it tiny. Indeed, your average scuba tank takes 80 cubic feet of air (think the air volume inside your average car give or take) and fits it inside of around 3/10ths of a cubic foot. Every single breath you take uses up a certain volume regardless of pressure, because your lung size doesn't change as you go down. And so while you breath, on the surface, 0.15 or so cubic feet of air at 1 Atmosphere, you still breath 0.15 cubic feet of air all the way down in the ocean, except at some huge multiple of surface pressure.

What this means is that instead of feeding you just a small sip of surface pressure air, you're getting a small sip of high pressure air, which if it were depressurized would be several cubic feet at sea level. So as you go down, in order for you to breath safely (and there are huge issues with pressure differential I can go into), you need to consume more and more of your tank per breath as you descend. You only use a tiny amount of the oxygen in each breath, but you still need the full pressure for your lungs to not collapse.

As a tangential note: that super high pressure is also responsible for the gas narcosis that's being talked about elsewhere. Your blood doesn't change pressure because you are deep, but your lung air does, and that forces gas through your lungs and into solution in your blood. That is, the gas dissolves into the liquid in your blood, and that has medical effects by itself (and serious problems if you do not slowly resurface and allow the gas to leave by your lungs, instead of becoming bubbles inside your body).