# Eli5 Underwater caves with air

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How are there caves deep underground with air pockets? How does the water not fill into it? And if a person “lived” down there would the breathable air run out?

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> And if a person “lived” down there would the breathable air run out?

Assuming there’s really no link to the outside, then yes, it would. You breathe in air, and breathe out carbon dioxide. Trees absorb carbon dioxide and produce breathable air, it’s why they’re called the lungs of the world. So in a tiny space like that, you’d eventually run out of air.

You can see this in action by lighting a candle and putting a glass over it. Fire consumes oxygen in order to burn, so when it runs out of oxygen in that small container, it’s extinguished.

> And if a person “lived” down there would the breathable air run out?

Assuming there’s really no link to the outside, then yes, it would. You breathe in air, and breathe out carbon dioxide. Trees absorb carbon dioxide and produce breathable air, it’s why they’re called the lungs of the world. So in a tiny space like that, you’d eventually run out of air.

You can see this in action by lighting a candle and putting a glass over it. Fire consumes oxygen in order to burn, so when it runs out of oxygen in that small container, it’s extinguished.

If the air has nowhere to escape to, then yeah it will be there. The thing is, depending on how deep it is, it is going to be at a higher and higher pressure than at sea level. The water pressure will try to push into the space, compressing the air until the pressures between the two are equal. That’s why in the movie Sphere, they have to breathe a high-pressure gas mix that is mostly helium, because of how deep they were, having the air pressure very high inside the habitat made it a lot safer from rupture.

If the air has nowhere to escape to, then yeah it will be there. The thing is, depending on how deep it is, it is going to be at a higher and higher pressure than at sea level. The water pressure will try to push into the space, compressing the air until the pressures between the two are equal. That’s why in the movie Sphere, they have to breathe a high-pressure gas mix that is mostly helium, because of how deep they were, having the air pressure very high inside the habitat made it a lot safer from rupture.

Take a bowl and put it into a bucket of water straight upside down. You’ll end up with an air pocket in the bowl and it won’t fill with water. It’s the same
Thing with an underwater cave, the air sits on top of the water and water can’t fill the space unless the air can escape

Take a bowl and put it into a bucket of water straight upside down. You’ll end up with an air pocket in the bowl and it won’t fill with water. It’s the same
Thing with an underwater cave, the air sits on top of the water and water can’t fill the space unless the air can escape

Basically underwater caves with air in them only have air in the parts that are above the water table. Caves are complex 3D structures, and while access might be restricted to underwater entry only, plenty of the cave complex may be above the water table. Small cracks and porous rock let air in from the surface.

In places with tides wierd things can happen that may pump air into unexpected places below the water table but you won’t have air too far down.

Some other caves that are only accessible with diving gear may actually be above the water table for the most part, but have parts that are mostly filled by flowing water that collects in the low parts of the caves.

Basically underwater caves with air in them only have air in the parts that are above the water table. Caves are complex 3D structures, and while access might be restricted to underwater entry only, plenty of the cave complex may be above the water table. Small cracks and porous rock let air in from the surface.

In places with tides wierd things can happen that may pump air into unexpected places below the water table but you won’t have air too far down.

Some other caves that are only accessible with diving gear may actually be above the water table for the most part, but have parts that are mostly filled by flowing water that collects in the low parts of the caves.