If yes, why don't they fill up with water, and can you breathe the air there? Like, it's not exactly atmosphere there, but an underwater cave with higher ceiling.

P.S. Possible that it has a different name than air pocket, but due to my limited knowledge of English, I have no idea.

Thanks in advance!


3 Answers 3


Yes, it is possible to have pockets of air underwater as long as there is something there to contain the pocket.

You can easily demonstrate this by turning a cup upside-down and submerging it in water. If you put a napkin in the bottom of the cup before you do this, the napkin stays dry.

If you do the above experiment, but dive down in a swimming pool with the cup, you will notice that the air pocket decreases in size as you dive, and returns to normal size as you surface. This indicates that no air is being lost. Instead, the air is being compressed.

These observations indicate that the air pocket does fill part way up with water, but does not disappear. This is because as the air is submerged, the water pressure on it increases. This shrinks the air pocket's volume until the pressure in the air is the same as the pressure in the water surrounding the air.

Yes, you can breathe air in an air pocket - it's normal air. However, you will exhaust the oxygen supply quickly if the air pocket is small.

  • 4
    $\begingroup$ I've tried to measure if deep enough underwater compressed air would have density > than one of water, which is virtually not compressible. So, it should be possible to have a pocket of compressed air free-floating in the water at 1000K (should be possible near Vulcans) & 1000Bar (11km underwater). In salt water temperature/pressure might be much lower. But obviously humans can't breath at such conditions :-D $\endgroup$ Aug 4, 2011 at 0:54
  • $\begingroup$ The air pocket does not "fill up with water". The cup fills up with water, and the air pocket becomes smaller. $\endgroup$
    – endolith
    Aug 14, 2011 at 22:42
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    $\begingroup$ Note that over time, the soluble gas molecules will slowly dissolve in the seawater. I don't know what time-scale this process takes place over, but I expect that underwater caves will not generally contain air pockets inside of them, even if they could geometrically. $\endgroup$ Aug 29, 2011 at 18:43
  • $\begingroup$ @Peter. And if they are old, I wouldn't expect the same relative concentrations of gases. It might be gas, but it might not be good to breathe. I could imagine a cave, where the air is renewed only at low tide, perhaps only a rare very low low tide. I wonder what sort of composition that air would have. Near the surf zone, is bubbles/foam are advected to your "bubble" they could also contribute. $\endgroup$ Aug 29, 2011 at 22:14
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    $\begingroup$ @Omega: The gasses dissolved in sea water are generally in equilibrium with the atmosphere, so if you had an underwater cave with an air pocket above sea level, it might end up having more or less the composition of the atmosphere (although I certainly wouldn't recommend testing this by trying to breathe it), but if you have an underwater cave with an air pocket below sea level, I suspect that all the gas will eventually dissolve. $\endgroup$ Aug 29, 2011 at 23:05

Yes, it can exist, and the air will be pressurized to the level of the pressure that the water column exerts, which is roughly 1 bar for 10 m of depth (slightly more in sea water, because of its higher density).

There even is a practical application for this, which is called a trompe. There's an article on Wikipedia here. The gist is that air is sucked down a shaft by falling water, separates from the water stream in a chamber down below, which is, effectively, an underwater cave, and is now under pressure so when you run a pipe to the top of the chamber and put a valve on it, you have a supply of pressurized air.

As for breathing the air, at some point, when the pressure is high enough, the different partial pressures of nitrogen and oxygen will mean that breathing our normal atmosphere will deposit too much nitrogen in your blood stream. This is a problem divers face, and the reason why they don't use ordinary air for breathing beyond a particular depth. (Actually, the effects are a bit more complex, and even the oxygen itself is not harmless at higher pressures. For further information on that, see the Wikipedia article on "nitrogen narcosis").


A man just survived 3 days in a boat underwater in a air pocket, read this: http://www.slate.com/articles/health_and_science/science/2013/06/harrison_okene_s_shipwreck_air_bubble_how_could_he_survive_underwater_for.html

Okene’s salvation—the air bubble—was trapped because the overturned boat acted as a sort of diving bell, the cup-shaped chambers that have transported explorers and workers into the depths for centuries. In the fourth century B.C., Aristotle described the contraptions as enabling “the divers to respire equally well by letting down a cauldron, for this does not fill with water, but retains the air, for it is forced straight down into the water.” Years later, diving bells called caissons helped 19th-century workers construct the Brooklyn Bridge (though many died in the process).

Whether in a bell or boat, trapped air rises to the top of a concave chamber. The only way it can escape is by diffusing through the water itself, one molecule at a time. Eventually this would happen, but Okene would have succumbed to thirst, hypothermia, or asphyxiation long before his air bubble diffused into the ocean.


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