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I read an article recently about gelatinous fish that live deep underwater, and if they were brought up to the surface would 'melt' due to the lower pressure not supporting their bodies.

This got me thinking, if a human were to go down to the bottom of the Marina trench if it were not underwater (so basically just a 11km deep; below sea level valley), would we be crushed by the pressure of all the extra air above us?

For clarity based on the comments: yes, all the water would be replaced with air. Not just fill with existing air flowing in from around.

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    $\begingroup$ Are you replacing the water with more air, or are you simply removing the water and allowing the existing air to flow downwards into the ocean basins? $\endgroup$ – probably_someone Nov 15 '18 at 14:40
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    $\begingroup$ Pressure itself wouldn't be your problem, it would be the oxygen itself that would be your primary issue en.wikipedia.org/wiki/Oxygen_toxicity $\endgroup$ – Triatticus Nov 15 '18 at 14:44
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    $\begingroup$ For full clarity, you'd be removing all the water and adding a bunch of extra air, so that the pressure at (the current) sea level remains the same? $\endgroup$ – Emilio Pisanty Nov 15 '18 at 14:51
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    $\begingroup$ They wouldn'd die from high air pressure, but they would certainly die from heat, as the temperature would be about 11km*6.5°C/km=71.5°C warmer than at the surface. $\endgroup$ – gerrit Nov 19 '18 at 11:25
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    $\begingroup$ @gerrit I'm really curious where that math comes from. I think that would be a good (and surprising) answer to the question. $\endgroup$ – JMac Nov 19 '18 at 20:56
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This would be equivalent to scuba diving at a depth of roughly 35 meters, or 115 feet. People do it routinely, it causes no health problems, and you don't need to breathe special, low-oxygen-and-nitrogen mixtures as you would in order to go much deeper. However, after spending much time at that pressure, you would need to decompress slowly in order to avoid forming dangerous nitrogen bubbles in your blood ("the bends"). You can check out a dive table to see how gradually you'd need to decompress in order to emerge safely from a given amount of time spent at this pressure.

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The typical lapse rate on Earth is about 6.5°C/km. That's why it may be a cozy 20°C at the surface but below -25°C at the summit of Mt. Everest.

Extrapolate that in the opposite direction, and the bottom of an ocean-free Mariana Trench would be almost 70°C warmer than the surface. Therefore, regardless of pressure, the temperature would be approaching 100°C, and be completely deadly. Even if that may be inaccurate depending on exactly what insolation the Mariana Trench gets and how circulation changes, the column of GHG above the Mariana Trench would be several times more than on current sea level. Valley bottoms can get very hot already today: Death Valley in the USA is less than 100 metre below sea level and temperatures reach above 50°C. It doesn't take much more heat than that to be more immediately deadly.

If you were to drill a 10 km hole on land, it would be deadly hot as well, not due to the lapse rate, but due to geothermal heat. The bottom of the 12 km deep Kola Superdeep Borehole was expected to be 100°C, but instead found to be 180°C. In deep underground mines within the Arctic Circle, miners can work in shirts and shorts year round (as far as temperature is concerned).

If the Mariana trench wasn't underwater, it means all the oceans have evaporated, which would only happen as a consequence of a runaway greenhouse effect, which means we're all dead already anywhere on Earth anyway. Or even if we succeed to make the water magically go somewhere else than the atmosphere, its absence would cause a very extreme climate, in particular at such deep and dry spots.

All of the above is a simplification, as the entire climate on Earth would dramatically change. For a more sophisticated answer (still simplified, but less so), one could run a climate model with the oceans switched off, and see what happens. I don't expect the bottom of the Mariana trench to be a pleasant location under those circumstances

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    $\begingroup$ Such numbers are not a plausible extrapolation. That lapse rate value is an average observed above ordinary Earth surface that is well exposed to solar radiation. Valleys are less exposed to solar radiation and thus much colder at the same height above sea. Lapse rate depends also on air convection. Introduction of a big and wide valley will change convection on its boundary. $\endgroup$ – Ján Lalinský Nov 20 '18 at 1:50
  • $\begingroup$ @JánLalinský The Mariana Trench may be a valley, the surrounding ocean basin is not, so even if it isn't 70°C warmer than the current sea level, even half as much of a temperature increase will be deadly. If the Mariana Trench wasn't underwater, nor would the rest of the Pacific Ocean. I need to think about whether I agree with your second comment, I suppose the temperature would redistribute globally, making the current sea level on average colder if all the oceans were dry. But also, the total column of GHG from -8km is much more than from 0 km. I do stand by my final paragraph, though. $\endgroup$ – gerrit Nov 20 '18 at 10:00
  • $\begingroup$ @JánLalinský Plus, there is yet another effect. If you were to drill a 10 km hole on land, it would be extremely hot there, not due to the Sun, but due to geothermal heat. $\endgroup$ – gerrit Nov 20 '18 at 10:02
  • $\begingroup$ @JánLalinský I don't know how steep the walls of the Mariana Trench are, but it's pretty wide and I expect it would get plenty of sunshine. Considering the temperatures in places like Death Valley or the bottom of the Grand Canyon, I'm not at all convinced by your "valley" argument. I have however reformulated my answer. A proper answer would take a good climate model. $\endgroup$ – gerrit Nov 20 '18 at 10:06
  • $\begingroup$ @JánLalinský Your comment on "maximum possible temperature (...) is determined by intensity of solar radiation" is incorrect, because it fails to consider the greenhouse effect. Venus is hotter than Mercury with much less solar radiation. Earth would be freezing without the greenhouse effect. We can theoretically make it almost arbitrarily hot by adding powerful GHG in abundance. I do think the temperatures at a hypothetically dry bottom of Mariana Trench would reach 100°C (even without extra GHG). That's only 43°C warmer than temperatures that have been measured at Death Valley, USA. $\endgroup$ – gerrit Nov 20 '18 at 10:13

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