Let's assume that this hypothetical alien crawled out of the identical primordial ooze to the point where they share significant genetic similarity with humans, and due to either evolution or outside intervention, evolved to be similar physiologically to humans if not in appearance. They have stronger muscles/bones, better lungs and circulatory system, etc. How far could they dive into our ocean before they couldn't withstand the pressure?

Assume that evolving on their home planet gave them efficient enough lungs that Oxygen/Nitrogen toxicity isn't an issue.

  • 1
    $\begingroup$ I'm dubious as to whether life on such a planet could crawl out of the ocean, much less grow trees that aren't bushes. $\endgroup$
    – RonJohn
    Nov 10, 2018 at 20:07
  • 1
    $\begingroup$ And even life could evolve on land, and grow a civilization that can reasonably contemplate space travel, they aren't going anywhere. space.stackexchange.com/a/17576/17697 $\endgroup$
    – RonJohn
    Nov 10, 2018 at 20:10
  • 2
    $\begingroup$ Oxygen toxicity is always going to be an issue, regardless of gravity; that is based on completely different issues. Also, what makes you think that the level of gravity a species has evolved in will be a major factor in how deep they could dive? (Lots of marine life has evolved, and live, in 1 g, on Earth.) If you can Edit to expand on the reasoning behind your question, we might be able to give you better answers. See also How to write the perfect question for several helpful hints on what makes a good Worldbuilding SE question. $\endgroup$
    – user
    Nov 10, 2018 at 20:13
  • $\begingroup$ Your presuppositions are all wrong. You can't have beings that are "physiologically similar to humans" and are able to withstand 10x more gravity without artificial exoskeletons. They would have to be built from different materials and would have different basis for biochemistry. They wouldn't share any genetic similarity to humans in the sense that they would have to have different compounds for that $\endgroup$
    – Gnudiff
    Nov 11, 2018 at 12:08
  • $\begingroup$ @Gnudiff "they would have to have different compounds for [genetics]" For that part, OP might be interested in my question Why would life on a different planet use DNA? $\endgroup$
    – user
    Nov 11, 2018 at 12:10

4 Answers 4


Such a being would need to wear an oxygen mask to breathe on Earth at sea level

An outrageous simplification is that your alien's homeworld sea level atmospheric pressure is 10 bar. This is outrageous because a LOT goes into atmospheric pressure, not just gravity, and you have not provided enough information to correctly calculate the atmospheric pressure on your planet. So, I'm assuming just as it's 10X gravity, it's also 10X pressure.

And this is worth a side note. Neither man nor alien doth dive by gravity alone. The equation is much more complicated than that. Indeed, I feel comfortable saying that gravity is only a minor component of the issue, but let's roll with what we've got.

10% of atmospheric pressure on Earth (0.1 bar) happens at about 20 Km in altitude. Mount Everest is only 8.8 Km, and it's a rare person who can obtain the summit without an oxygen mask, and nobody can stay there without one (it's more complicated than that... they need to get back to the last camp before sunset for a lot of reasons, temperature being only one of them).

So, you're alien is standing on the California coast, feeling very uncomfortable in the low pressure, and hating every moment of breathing in his exosuit — which he needs to keep from suffering low-pressure bruising and other nasty affects of not having the proper pressure on his/her skin.1

He needs to be 0.1 Km underwater just to breathe comfortably

Diving to 99.55 m (call it 0.1 Km for ease-of-use) gives us an equivalent atmospheric pressure in seawater. He needs scuba gear,2 but at least he can breathe easily. Like Aquaman,3 he's very comfortable at this depth — other than it's hard to see.4 Artificial lights are your friend.

So, how low can he go?

Ahmed Gabr holds the world scuba diving record at 332.35 m (oh, call it 0.3 Km). Using that link above for pressure, 0.3 Km is a whopping 30.14 bar! That's 30X the "baseline" pressure! This suggests5 your alien could handle (if he was world-record material, we'll get to that momentarily) 300 bar.6

That happens on Earth at 2,986 meters below sea level. Let's call it 3 Km.

What if we're not world-record material?

To be honest, though, that feels like it stretches credibility. No matter how well you set up the science of your story, people need to suspend their disbelief — and that feels like it stretches it mighty thin. But, we're not all world-record holders, either.

  • I'm comfortable believing your average alien can dive to 1.0 Km oceanic depth.7

  • In an emergency, I'm comfortable believing your average alien could hit 1.5 Km.

  • If your alien is exceptionally athletic and very practiced in deep diving, I'm comfortable believing 2.0 Km.8

1Remember, he's/she's/it's designed to keep the blood in against 10 bar. Standing on that beach... his/her blood really wants to get out. Oh, yes... it wants to get out something awful. It's the difference between submarines and space ships. But that's a story for another day.

2Because, unlike Aquaman, your alien doesn't have gills. At least you haven't mentioned gills, so I'm going for broke on this one.

3Except for the gills....

4 * Even Aquaman can't see at those depths, no matter what Marvel comics says.

5This is where I make the most wild supposition in my entire thought experiment. Just because our world-record holder can withstand 30X doesn't mean your alien can. There is a horrific number of variables actually in play, and we're only dealing with a handful. So, the goal here isn't 100% scientific accuracy (thank you for not using the tag), but believability.


7This assumes your alien has the physical strength to swim straight down 1.0 Km. Or, perhaps more accurately, to swim back up. Or to carry the equipment to get to that depth and stay there for any relevant length of time. Just because he/she can withstand that depth doesn't mean he/she can actually get there. That's a long distance underwater.

8Which is still above the 11 Km world-record holding dive by the Bathyscaphe Triest, so we shouldn't need Dr. Who to save us from what are most certainly Sea Devils. Cheers.

  • $\begingroup$ His exosuit is going to be a pressure vessel! $\endgroup$
    – RonJohn
    Nov 11, 2018 at 1:40

Insufficient data.

Their ability to withstand pressure doesn't depend on their ability to withstand surface gravity unsupported by buoyancy. Native Earthlings, all developing under the same gravitational field, have wildly different abilities to withstand both high and low pressures.

The argument that higher gravity equals higher atmospheric pressure automatically just doesn't work. Even with identical atmospheric composition, there is no fundamental reason an otherwise Earthlike planet couldn't have one tenth or ten times as much air--how much you end up with is basically a crap-shoot, related to what combination of materials randomly ends up accreting onto the planet during formation, and thus so is the final atmospheric pressure on the planet's surface.

So, how deep could they dive? As deep as you want them to. If you want to know how ten times greater native atmospheric pressure would affect them... well, then ask that.

  • Ptot = Patm + Pfluid
  • Pfluid = Ptot - Patm
  • Pfluid = 10atm - [(1.01E5 Pa)(9.86923E-6 atm/Pa)]
  • Pfluid = 9 atm
  • Pfluid = rgh
  • g = 9.8 m/s2
  • r = 1.03e3 kg/m3 (density of seawater)
  • h = Pfluid/rg
  • h = (9 atm)(101325.0 N/m2)/atm)/[(1.03e3 kg/m3)(9.8 m/s2)]
  • = 90.3 m

(if my math is right)



I feel the simplest solution to the OP's apparent problem would simply be to have the atmosphere have 1/10th the contents (disregarding differences in the planet's mass) while maintaining Earth-like percentages. A superior respiratory system would be needed to take in the necessary oxygen and certain flora would need to be far more efficient at breaking down CO2, but at least the alien wouldn't explode the moment they stepped out from a spacecraft pressurised to their planet's BAR into Earth's atmosphere.

Furthermore, reducing the percentage of Oxygen in the atmosphere proportionately to increases of Atmospheric Pressure should allow OP's alien to dive further, but again exceeding a partial pressure of 1.6 BAR would result in Oxygen Toxicity.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .