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Question inspired by Made in Abyss.

So for whatever reason, our world has a large crater/hole tens of kilometers in diameter and maybe 100 km deep (maybe our world's crust is a little thicker, maybe the hole goes past the crust). This hole is home to lost technologies and an exotic ecology, which tempts explorers and divers of all kinds.

Humans get altitude sickness climbing mountains, and diver's disease from underwater pressure and rapid ascension. In what ways can we get sick from going down into the hole? What are the factors that causes these sicknesses?

For the sake of the question, assume the ecology isn't the cause of any of these sicknesses. We're interested in the change in environment and altitude. Bonus cookies if you can provide ways to avoid such ailments.

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closed as off-topic by Vincent, Aify, sphennings, L.Dutch, Mołot Aug 4 '17 at 8:43

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question does not appear to be about worldbuilding, within the scope defined in the help center." – Vincent, Aify, sphennings, L.Dutch, Mołot
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ outsideonline.com/1922711/raising-dead long but almost certainly worth your time. Since it is rather sad please only read f you are older than 16 or so $\endgroup$ – Raditz_35 Aug 3 '17 at 17:10
  • $\begingroup$ Don't forget that some people get acrophobia, an irrational fear of heights (often causing vertigo), and it's possible to have similar triggered by being below ground level too, although I can't find a specific term in a quick search. I can't handle being in underground spaces at all, but have no problems with confined spaces - that hole sounds awful to me! $\endgroup$ – Matthew Aug 3 '17 at 19:07
  • $\begingroup$ pressure of the surrounding rock will squeeze the hole shut if it is that deep. the limit on our deepest drills is due to how fast the hole squeezes shut around the drill. making the hole wider only makes the problem worse. Another factor is heat, at that depth, the rock is at more than a thousand degrees celsius. $\endgroup$ – John Aug 3 '17 at 23:10
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    $\begingroup$ I don't think you can have a hole 100km deep on Earth without violating basic physics. The rock is under more pressure the deeper you go. At 100km below surface any hole would get squashed together immediately because of the 100km. of rock above that layer. $\endgroup$ – Gnudiff Aug 4 '17 at 5:46
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    $\begingroup$ This is also ignoring the 25°C per km you get when going deep in the earth I take it? $\endgroup$ – corsiKa Aug 4 '17 at 7:39
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There are many issues that arise. The body is not designed for high pressures.

As an example, as you go lower, you have to breathe air which has a smaller and smaller concentrations of oxygen. This is because if the partial pressure of oxygen rises too high (1.4-1.6atm), it becomes toxic and kills you.

There's nitrogen narcosis. Nitrogen has a narcotic like effect on people at depth (20-30m, depending on the individual), and deep see diving is not a healthy place to suffer narcotic effects.

Going below 150m with helium in your breathing mixture comes with the risk of High Pressure Nervous Syndrome, which can cause all sorts of problems. In fact, many commercial divers will breathe trimix, a mixture of oxygen, helium, and nitrogen. They introduce the nitrogen, and the risks of nitrogen narcosis, to decrease the partial pressures of the helium to offset HPNS. These mixes are often adjusted on the fly, depending on your particular depth at that time.

Around 2-4km deep, we believe we will run into issues with helium narcosis. Helium is considered to be the least narcotic gas known, and models suggest it becomes fatal at those depths. At this time, nobody knows if there's a way past that boundary or not.

The human body was not designed to survive the pressures 100km down. It's chemical engines are simply being run outside of their specifications at that point.

Edit: All of this was based on the idea that the hole was in water. If the hole was in air, obviously the pressures accumulate much slower. This calculator suggests that at 100km, we would experience an air pressure of 500atm. That's roughly equivalent to 5km depths of water. So if that calculator is correct, the bottom of the hole will still be just outside of the limits of the human body's susceptibility to helium narcosis.

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  • $\begingroup$ Any idea if all this can be avoided with air tanks? $\endgroup$ – Jim Wu Aug 3 '17 at 17:10
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    $\begingroup$ Air is the least helpful gas to breathe. It's toxic well before 100m due to oxygen toxicity. Divers typically mix in lots of helium to minimize these effects, but that's where we see the issues with HPNS show up. Of course, very few divers dive that far down because the decompression stops can literally be hours, so there's a lot of risk. $\endgroup$ – Cort Ammon Aug 3 '17 at 17:13
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    $\begingroup$ I think you describe the effects of air at the pressure of hundreds or thousands of metres of water. The OP asks about air at the pressure of tens of kilometres of air. $\endgroup$ – o.m. Aug 3 '17 at 17:16
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    $\begingroup$ @o.m I ran the numbers and made an edit. The result still doesn't look good, but it leaves a bit more room for human error in my calculations. I don't know how to validate whether an air pressure calculator is valid that many km down! $\endgroup$ – Cort Ammon Aug 3 '17 at 17:22
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    $\begingroup$ The problem is that you're simply not going to get a hole that deep. 1) The same strength of materials factors that limit Earthly mountains to about 50K ft work the other way around. (I don't think it's a coincidence that the deepest ocean trenches are about as deep as the tallest mountains are tall.) 2) On a roughly Earthlike world, the hole will fill with water. $\endgroup$ – jamesqf Aug 3 '17 at 18:10
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This sickness is very common for divers. If we stay in the air (rather than water), pressure increasing much less, so we need to go much deeper to experience the same problems.

  1. Decompression sickness is common when humans return from high to low pressure too quickly. Gases that were dissolved in blood at high pressure start to boil, and this may cause serious health damage.
  2. Oxygen toxicity. While decompression sickness can easily be avoided, high air pressure is not. Under higher pressure, amount of oxygen in each breath is higher, and here lies the problem. Actually, just 50% increase in pressure may cause the oxygen to become toxic.
  3. Nitrogen narcosis also occurs at high pressure. Nitrogen, while mostly inert with respect to human body, at high pressure (typically 5+ bar starting at about 2 bar) affects our brains, causing a number of possible effects, including death.
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  • $\begingroup$ May want to check on that partial pressure of Oxygen for toxicity. It is possible to dive (in the extreme) to 100m depth salt water on air. That's 11 atmospheres, The partial pressure would be over 2. A 50% increase is only 5m or about 16 feet. $\endgroup$ – mckenzm Aug 3 '17 at 20:50
  • $\begingroup$ "sea-level air has a partial pressure of oxygen of 0.21 bar (21 kPa) whereas toxicity does not occur below 0.3 bar (30 kPa)" archive.rubicon-foundation.org/xmlui/handle/123456789/3863 $\endgroup$ – Alexander Aug 3 '17 at 20:57
  • $\begingroup$ See worldbuilding.stackexchange.com/questions/21079/… and other worldbuilding questions for info on O2 partial pressures that are survivable for a few minutes vs. a day vs. no long-term effects. $\endgroup$ – Peter Cordes Aug 4 '17 at 5:52
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Anything that's a concern for the mining industry should also be yours.

Long term exposure to radon that has accumulated in the hole will give you lung cancer.

Elevated levels of Carbon Dioxide, Carbon Monoxide, Hydrogen, Hydrogen Sulfide, Oxygen, Methane, Nitrogen, Nitrogen Dioxide, and Sulfur Dioxide are also hazardous, and are the main concerns of miners. This link contains spread sheets for their effects at given concentrations. (miningquiz.com/download/minegases.htm)

I'm unsure exactly which of those increases solely with depth (other than radon). Any that are heavier than air will accumulate in higher concentrations the further down you go.

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    $\begingroup$ These are not going to be a problem. Mines are confined spaces, with limited air interchange (unless mechanically ventilated). This hole, if it were physically possible, would be "tens of kilometers" wide, so it would be part of the atmospheric circulation, and little more subject to buildup of gasses than say Death Valley. $\endgroup$ – jamesqf Aug 4 '17 at 5:49
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    $\begingroup$ @jamesqf - Sections of the Valley's valleys are about +10k wide, but its deepest point is only -86m, penned in by ranges that are ~3K. If it were -103k K deep (instead of ~3086m), and still just ~10K wide, I think the air would be stratified (especially if the temp range was smaller), at least occasionally. This place would need signs that say, Bad Air Day. Enter At Your Own Risk. $\endgroup$ – Mazura Aug 4 '17 at 17:37
  • $\begingroup$ Thankfully, if it were physically possible is discussed in the other answers. Weather can do some weird stuff; look at Jupiter. "I don't think you can have a hole 100km deep on Earth without violating basic physics." Perhaps not, unless it becomes a giant red spot, and the outward pressure from the vortex keeps it from collapsing, considering IMO that's probably how the hole got to be so big in the first place. Either way, don't go there. $\endgroup$ – Mazura Aug 4 '17 at 17:41
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I'm not 100% on the numbers, but the temperature increases by as much as 30K per km of depth. This means that as you descended, by 5km (probably not a huge pressure difference, only half as deep as Everest is high) the temp would be that of an oven. I think that your explorers would be cooked before they had to worry hugely about pressure differences. So the ailment might well be good old heat stroke.

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A lot of the answers have assumed that the deeper you go, the heavier the air pressure. This is not always that case, because going down means closer to the earth's core, means higher temperature, means lighter air. So, you probably have to choose the sweet spot between pressure related issues and temperature related issues.

Lower down, you probably have water running into the hole, so at the bottom there may be a lake. Or if it's boiling hot, maybe the water vaporizes.

We can go pretty far underwater, and every 33ft (34ft in fresh water) leads to another atmosphere of pressure (water doesn't compress like a gas would, so is far more stable). So we can extrapolate a lot of deep sea diving problems and solutions, most especially about breathing under pressure.

A rigid ship or suit might be the easiest thing to deal with. It hides a lot of the issues specifically, and still gives you a "dangerous" feel. Alternatively, you probably want heat reflective clothing and a helium-oxygen blend to breathe; and to have plenty of time for acclimating as you go down/up.

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    $\begingroup$ That's "every 33 feet" is another "atmosphere" of pressure, not meters. (I think 2 characters is too small an edit for someone other than the writer) $\endgroup$ – Xen2050 Aug 4 '17 at 3:52
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One thing that also might affect somebody travelling down that deep is the change in gravity. As you get closer to the bottom, depending on how large your world is, there's a good chance that you'll experience a noticeable change in gravity. At depths (in caves for example) or heights (on mountains for example) of just a few kilometers max on Earth this change is negligible, but based on your world at say 100km depth, this change could definitely be felt.

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The bends/ decompression sickness might count. In this case gas forms bubbles that will mess you up. https://en.m.wikipedia.org/wiki/Decompression_sickness

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