Underwater Moon Pool (Wet Porch) - Is This Really a Thing?

Added: 2026-05-12

[00:00:00]Moon pools. These are things that show up in actually a surprising number of underwater movies and the idea is it's an open pool of water inside some sort of undersea habitat. And honestly, I think these show up in movies just because they look really cool visually, artistically. You've got this pool of shimmering reflective open water in the middle of a very dirty, grimy, industrial sort of mechanical space. And they're neat. And of course, they open up a lot of interesting plot elements.

[00:00:30]But the question is are they real? Like how can you actually have an underwater building with a big hole in it open to the ocean and not have all the water come in and all the air go out and everyone die? Well, it's actually really simple. Uh let me just show you.

[00:00:47]All right, so let's say this is our ocean, this bucket of water, and this is our underwater habitat turned upside down. Well, if I take this and I just drop it into the water and hold it down, why doesn't it fill up with water? Well, the answer is the air is stuck in there.

[00:01:05]And what has happened here is as I push this glass deeper and deeper down into the water, the air the pressure pushing up into the glass is going to increase.

[00:01:15]The volume of air is going to decrease, but the air is going to get pressurized to the exact same level as the water outside. So, as long as there's nowhere out for the air to go, it just stays there.

[00:01:28]All right, so super easy, right? As long as the air's trapped in that hab station hab station. Uh you can just put it underwater and the air will stay there and it can't get out. Uh now of course, this is in part because the hole is on the bottom of the station. That's very important. If you were to take this hab station and flip it 180 degrees so the hole was on top, well, then the air can get out and the water is going to come in because the air is less dense than the water and then everyone dies. So, don't flip your hab station upside down. That should be pretty easy. Uh However, there are some implications to to this setup that movies don't always explain and that they don't always get right either. So, the big one is by definition, if you have an open moon pool in a submerged habitat building, then the air pressure inside the habitat is the same as the water pressure outside. This means for example, uh well, it means your divers are all in what's really going to be considered saturation diving. The the habitat I shouldn't say divers. The inhabitants of your station are breathing compressed air just as if they were diving outside.

[00:02:33]And that means they cannot just walk out to the surface or they will have horrible decompression sickness, the bends, because they're breathing either they're breathing a lot of well, they're getting a lot of nitrogen absorbed into their bodies because they're essentially living in a high-pressure environment. Now, when that happens, there are a couple other follow-on effects. Typically, you're not going to be breathing regular air because regular air has too much oxygen when you pressurize it at depth. And if you don't know what I'm talking about, I will link to my video on oxygen toxicity uh or gas toxicities and uh check that one out. But [snorts] assuming you recognize the issues with oxygen toxicity, you're going to be breathing in a habitat like that a different gas mixture. Typically, it's going to involve a lot of helium as an inert neutral gas. Well, aside from all of its other properties or lack of properties, helium is a very lightweight gas. And so it affects your vocal cords differently. If you've ever sucked in helium from a party balloon and then talked and sounded like a hyperactive chipmunk, that's what divers in saturation environments sound like because that's the sort of gas that they're breathing in.

[00:03:55]Uh to varying degrees because it depend you know, the the gas gets denser as it gets pressurized. But movies virtually never actually address this. And the really good ones will address it and just hand-wave it away. Like, "Okay, we're going down to to depth, so put on your special voice modulator so you don't sound like a hyperactive chipmunk." And then they can just do the rest of the movie with normal voices because if they did actually have everyone sound like an actual saturation diver, no one would be able to take the movie seriously. So, normally, that would kind of be a clue that you're not in just a regular normal ambient pressure environment. Now, this takes us to one of the other questions that I think a lot of people will have, which is it seems really bizarre that you could be underwater walking around in in an air-filled habitat with this open moon pool and just jump into the moon pool and swim outside somewhere because a lot of the movies that have moon pools will use that as some sort of plot element. They certainly do in The Abyss, which is probably the best of these. Uh can you really do that? The answer is yes, you can really do that because uh you're already you're essentially your body is is at the same pressure as the water outside. So, it's no different really than free diving with the important exception that it if you're in this sort of environment, it's probably really cold. There are not a whole lot of deep water aquatic habitats that are at nice, you know, 85-degree tropical water temperatures. No, they're all just barely above freezing. And so the real hazard from jumping in and swimming like that without a scuba rig is you're going to freeze to death or even with a scuba rig. But if you're not in a good environment suit, a drysuit typically, you're going to get hypothermia and die pretty quickly.

[00:05:38]And sometimes the good movies do take that into account.

[00:05:43]So, yes, open moon pool is a thing. Yes, you can jump right into it and dive. Um One of the hazards of a moon pool, I would imagine, um not an engineer who has ever actually worked on a on construction or maintenance of a thing like this, but if you get a leak in the habitat module, if there is a way for way for air to get out, that's going to disrupt the balance of pressure inside the system. And instead of water spraying in through the leak, what's going to happen is uh the moon pool is going to come up real fast. If you were to just hypothetically open a hatch in the top of one of these habs underwater, uh air's going to go rushing out the hatch upward and the easiest way for water to get back in is going to be through that hole in the bottom. And the moon pool is just going to explode upward until the air pressure equalizes.

[00:06:33]So, if the hatch stays open and all the air comes out, then the moon pool is very quickly going to fill the entire structure with water.

[00:06:40]Uh for this reason, it makes sense to have airlocks in a moon pool built in a moon pool room so that if something bad happens in the rest of the station, you can at least uh you know, you can section off this literal hole in the structure in the bottom of the structure. That said, those airlocks are there kind of as think of them as the safety uh um doors in ships.

[00:07:08]You'll never have a difference in pressure between one side and the other unless you have had some sort of catastrophic issue. So, you don't need to go through like a pressure change to go through the airlock into the moon pool. That defeats the purpose of having a moon pool. That would be just an airlock. And that implies that the rest of the station is uh being held at a lower pressure than than the ambient depth that you're at.

[00:07:34]Another interesting issue that this brings up is the use of like armored dive suits, the sort of suit that is theoretically a one-atmosphere suit so that you don't have any compression issues in the suit. You know, the the big, bulky, articulated, hard-shelled diving environment suits. Well, your divers are already at depth. Your your habitat occupants are already at depth.

[00:07:58]They're already acclimated to this pressure.

[00:08:01]There's not really any need to have an armored suit cuz the whole point of the armored suit is to prevent you from having to experience the pressure and the compressed gas uh ventilation of being at depth. So, if you're already there, just just a drysuit or a wetsuit is is going to be perfectly adequate. Now, a lot of movies are going to give you armored armored clunky dive exposure suits just because they also again look really cool. But if you have a moon pool, I would think it's pretty unusual that you would actually have any benefit from a big, expensive, clunky suit like that.

[00:08:36]So, in movies, you'll normally see these moon pools in really, you know, deep water, foreboding places and usually around monsters and aliens and mutated giant sharks. But that's just cuz where it's cool. That's where it's cool to set the movies. In reality, you can use these things and in fact, they're probably much more practically useful in much shallower, much simpler dive conditions. For example, Florida International University has a habitat off Key Largo in I think about 50 ft of water that is accessed by exactly this sort of open moon pool. It doesn't look quite as as artistically gorgeous as you know, the moon pool in Abyss. Uh but uh if you want to check it out, I would suggest taking a look at Jonathan Bird's video. Uh about 10 years ago, he did a short video tour of the Aquarius lab that's really cool and shows you exactly this sort of of open floor hatch moon pool as an entrance. So, I'll link to that down in the description text below.

[00:09:35]Check it out. Hopefully, you guys enjoyed the video. Thanks for watching.