Submersible Spaceship

"Yamato, Launch!"

Ensign Mayweather: You did say most of this thing's power was routed to structural integrity...
Commander Tucker: And you said we could probably fly it inside a gas giant. [Beat] Hell, it's only water!

— Star Trek: Enterprise ("Azati Prime")

The inverse of If It Swims, It Flies.

Superficially, spaceships and submarines have a lot in common. They're both essentially giant containers of air, intended to keep their biological crew alive in environments that would normally kill them instantly. Getting in or out while underway requires the use of an airlock, and both operate in a three-dimensional environment that can be confusing to someone used to 2-D Space.

The differences, however, are quite striking. The first and most important is that, despite their superficial similarities, space and the deep ocean are pretty much the exact opposite in terms of atmospheric pressure. A spaceship is designed to maintain a surface-level atmosphere in an environment with no atmosphere at all — it has to hold its pressure inside itself so that the ship doesn't burst open and leak its air into space. A submarine, by contrast, maintains a surface-level atmosphere in an environment of extreme pressure — it's designed to direct the pressure outward so that the vehicle and its crew aren't crushed like an empty beer can by the weight of the water around them.

Furthermore, a spacecraft's propulsion system is designed to work in a vacuum. Rockets and comparable thrusters won't necessarily work underwater — especially if they have openings that would allow water to pour in. A submarine, by contrast, will likely have a propulsion system that requires a surrounding fluid and can't work in a vacuum, such as propellers.

Despite this, the rules of Space Is an Ocean often allow spaceships to work just as well underwater, sometimes with the justification of advanced hull design or some kind of Deflector Shields. After all, many fictional spaceships are designed to endure crushing acceleration forces and tank massive amounts of combat damage; who's to say that whatever protects it from Energy Weapons and Antimatter bombs can't hold off the ocean for a little while? And whatever kind of engine allows Faster-Than-Light Travel is clearly very different from a regular rocket, so maybe it doesn't care about things like exhaust vents or conservation of momentum.

A common variant is to have a spaceship floating through the dense atmosphere of a gas giant, relying on updrafts and thick cloud layers to keep it aloft. Air and liquids are both fluids, after all, so it doesn't seem like that much of a stretch to compare this environment to an ocean. In practice, however, you'd have to get pretty deep inside the atmosphere for the air to get thick enough for this, and it's likely you'd be crushed by the pressure (or melted into nothingness by the planet's heat) by the time you did so.

See also Sub Story. As mentioned above, If It Swims, It Flies is this trope's opposite: a submarine that can also fly in space.

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Examples:

Anime & Manga

• Martian Successor Nadesico: The Nadesico can submerge, as in the first episode it launches from a concealed submarine hangar.
• While the Archangel wasn't capable of operating underwater in Mobile Suit Gundam SEED, by the time of Mobile Suit Gundam SEED Destiny, she has been specifically upgraded for that, in order to make it easier for the Clyne Faction to hide an entire space battleship on Earth.
• In the original Science Ninja Team Gatchaman, although mostly flying through the earth's atmosphere, the team’s Cool Starship, "The God Phoenix", is technically a spaceship, and has ventured out into space, even to the vicinity of the moon at least once. The God Phoenix is also able to travel under water, and after the introduction of the secret base at ‘crescent moon reef’ it is actually launched from an underwater hangar that gets flooded before opening to the sea. After which the God Phoenix sails out, switches on its rocket engines, and breaks through the water surface, missile style.
• Space Battleship Yamato and Space Battleship Yamato 2199: The titular Yamato can function as a spaceship, oceangoing vessel, and submarine, as in both versions it hides in the seas of a terraformed Pluto to trick the Gamilon forces into believing it's been sunk.

Fan Works

• With This Ring: The Team finds an intact and functional spaceship at the bottom of the ocean, built by the Guardians of the Universe. It's been down there for so many thousands of years that it's almost entirely out of power, but otherwise has suffered no ill effects, and once Paul recharges it a bit, it's able to fly back into space and even go to warp speed.

Films — Animated

• Titan A.E.: While the Valkyrie is visiting the broken planet Sessharim, the landing party come under attack by a squadron of Drej fighters. Cale, Akima and Korso take to a watercraft to escape them, but one craft dives into the water, where it shadows the surface craft's movements. This same Drej craft succeeds in capturing Cale and Akima, and takes them to the Drej mothership.

Films — Live-Action

• The titular spaceship of Event Horizon is adrift in the clouds of Neptune. It's said to be in a decaying orbit, but if it was that low in the atmosphere it must already have been operating in the equivalent of an ocean for some time.
• Star Trek Into Darkness: At the beginning of the film, whilst hiding their existence from a pre-warp civilization, the crew successfully manage to hide the Enterprise in a nearby ocean. invokedWhy they had to do this is never explained, but the ship holds together with no problems and is able to fly out of the water when they need to leave.

Literature

• Animorphs: The team steal a Bug fighter (a Yeerk spaceship that looks like a legless cockroach the size of a school bus) and use it to gain access to a Yeerk facility, Ax noting that ships can function underwater for a limited time.
• One of the short stories in Poul Anderson's Cold Victory is about a spaceship that suffers an attempted hijacking, and in the process, loses a large amount of its fuel. Lacking enough power to return to Earth, the crew instead direct the ship into a decaying orbit around Jupiter, and eventually come to rest within its atmosphere, at a depth where the gases are so dense the ship can float "like a bubble in a densitometer." They have to deal with weighing several times as much as usual, but the ship is otherwise unharmed and functional while waiting for rescue.
• In the prologue to Horus Rising, some of the fake Emperor's six hundred warships rise out of his planet's oceans to confront Horus' fleet.
• Justified in the Hyperion Cantos. The spaceship used by the main characters was designed to be able to survive the heat of a red giant's outer layers. Therefore, it was able to stay underwater for a few years without any problem.
• In the Lensman novel Galactic Patrol, it's mentioned that the "big teardrop" spaceships sink in water but are readily maneuverable underwater. Later on, there's a brief report that a fleeing spaceship "dove into the deepest ocean of Corvina II, in the depths of which all rays are useless."
• Justified in the Republic Commando Series. Kal Skirata spends some time tracking a runaway Kaminoan scientist, hoping she can reverse the clone troopers' accelerated aging. Since the Kaminoans are amphibious, he purchases a Mon Calamari-built shuttle for the purpose that's designed to double as a submarine.
• Skylark Series: Early atomic-powered spacecraft, which have no exhaust trail and can change direction rapidly, tend to be heavily armored spheres, and by book two, they're using Fantasy Metals. The pressure of the ocean is nothing compared to what they regularly face in combat. When the Skylark visits the world of Dasor, which has very little landmass above sea level, Margaret worries for a moment before remembering that the ship works just as well in water as in space.
• Subverted in "Space Opera" by Ray Russell. The villain is a Galactic Conqueror, and the only planet he can't conquer is an underwater civilization, because he doesn't have a material both light enough for a spaceship and durable enough for ocean depths. He manages to capture a scientist who, after threats to his daughter, does create such a material. However, while the villain tests it for durability, heat resistance, and other qualities, he forgets to test whether it is waterproof.
• In H. Beam Piper's Space Viking, starships don't usually land in water, but hiding one at the bottom of an ocean to ambush an enemy is a proposed tactic. The protagonist notes that at one point he was planning to ambush his adversary that way, and that his enemy's ship could in turn easily be hiding under "a thousand feet of water" where orbital scans couldn't detect it.

Live-Action TV

• Farscape:
  • Deconstructed in "I, E.T.", in which Moya evades detection by the Peacekeepers by landing on a planet and sinking beneath the surface of a swamp. Leviathans aren't normally meant to land at the best of times — much less swim — and the fact that Rygel has to perform surgery on Moya while she's submerged only ends up exacerbating the situation. As a result, Moya very nearly dies.
  • In "The Peacekeeper Wars Part 2," Moya is forced to hide under the surface of an ocean world in order to escape an attack by the Scarrans and stay there while the crew rescue the locals. Unfortunately, as Sikozu observes, there's a substantial difference between the vacuum of space and the bottom of the ocean, and Moya begins taking on water on several tiers. Plus, the transport pods that will take them to the surface aren't meant to serve as submersibles either, so the crew have to modify it to that end. Moya survives the escapade once again, but only barely.
• The Orville: In the Bad Future created in "The Road Not Taken", the Orville was shot down by the Moclans during the Battle of Earth and sank to the bottom of the Mariana Trench. The alternate Orville crew are able to reach her by diving a shuttle into the ocean and find her intact enough to repair and bring to the surface.
• In Red Dwarf, the exploratory ship Starbug can do this. Visiting Sdrawkcab Htrae, the crew park it in a lake to escape notice; on a waterworld, they are able to dive deep and maneuver underwater to dock with a long-lost starship that crashed there.
• Stargate Atlantis: The eponymous lost city of Atlantis turns out to be a city-sized spaceship that was deliberately abandoned and submerged to the bottom of an ocean. Justified by the fact that the city's energy shield is what protects it from the ocean pressure while underwater, and what prevents air from leaving the city while in space. However, after the Atlantis expedition finds the city, they quickly discover that the shield's power is nearly depleted and that its failure threatens to cause a massive flood.
• Star Trek:
  • Star Trek: Enterprise:
    • In the pilot, "Broken Bow", Enterprise enters the atmosphere of a gas giant to search for a Suliban base hidden beneath a layer of liquid phosphorous. Aside from some mild turbulence, the ship holds together just fine.
      Archer: It's just a little bad weather!
    • In "Sleeping Dogs", Enterprise finds a wrecked Klingon warship adrift in the atmosphere of a gas giant, and has to try to bring it back into orbit without accidentally dropping it further where it'll be crushed.
    • In "Hatchery", the crew recovers a wrecked Xindi-Insectoid shuttle that has almost all of its power devoted to structural integrity, to which Trip speculates that it could survive a trip through a gas giant. In the following episode, "Azati Prime", he and Travis use it to search the ocean of the titular planet for the Xindi superweapon, which is being built at an Underwater Base.
  • Star Trek: Deep Space Nine: Similar to "Sleeping Dogs", mentioned above, "Starship Down" has the Defiant trying to rescue a Karemma ship trapped in the atmosphere of a gas giant, while simultaneously dodging Jem'Hadar fighters that are hunting for both ships. invokedThe original script had this taking place in an actual ocean, with the immense pressure threatening to crush the Defiant before it could restore power and escape.
  • Star Trek: Voyager:
    • In "Scorpion, Part II", Voyager gets thrown into "fluidic space", a dimension that consists of nothing but green liquid in all directions. The ship is undamaged, although this may be somewhat Justified since it's a different universe with different physical laws.
    • In "Thirty Days", the Delta Flyer (after a small refit) is sent to probe beneath an alien ocean world — that is, a globe of water about 1,200km in diameter — to find out what's causing the force field holding it together to fail.
• UFO (1970): One of the alien spaceships manages to dodge Earth's defenses, and is hiding at the bottom of the English Channel. There, it activates some kind of Mind Control device that awakens Sleeper Agents that attempt to sabotage or sunder SHADO Control. Another alien spaceship hides at the bottom of a lake while its pilot starts abducting cute women to serve as host bodies.

Tabletop Games

• Classic Traveller, Supplement 7 Traders and Gunboats. System defense boats are built to survive the intense atmospheric pressure of gas giant planets, so they can easily handle the pressure of ocean depths as well. They are often stationed in planetary oceans because the water overhead protects them from being detected by enemies.

Video Games

• The API from Cosmic Star Heroine use their spaceships to break into the underwater terrorist HQ where Alyssa and co. were staying at the moment. This presents an opportunity to capture one of those ships, treating the player to the sight of a spaceship, now under Alyssa's command, triumphantly rising from the sea and into space.
• Honkai Impact 3rd: St. Freya's Hyperion, an airship that can fly to outer space, gets to dive underwater into the Mariana Trench in chapter 10 in order to get into Anti-Entropy's underwater base.
• Mass Effect 3: Mentioned in the Leviathan DLC. While the characters never have the chance to put it into practice, Lt. Cortez explains that the Kodiak shuttle is specced for up to a thousand times Earth's atmospheric pressure, so it is capable of deep-sea dives if the situation calls for it.
• Stellaris: One common early-game anomaly has the player's science ship find a derelict alien cruiser adrift in the atmosphere of a gas giant. The player has the option of trying to bring it back into orbit (and adding it to their navy) without accidentally dropping it further into the atmosphere where it'll be crushed.
• Fairly universal in the Super Robot Wars franchise; almost all (space) battleships can submerge in games that allow it. The inverse is also true, with ships that are submarines in their home series being fully capable spaceships when the story leaves the atmosphere.
• The Tenno dropships in Warframe aren't just capable of travelling through Earth-like atmospheres or vacuum, they can also dive into deep seas to break into Grineer underwater labs or perforate Jupiter's thick atmosphere to touch down on Corpus platforms. The archwings, similarly designed for space and atmospheric flight, can also function underwater, just not as swiftly.

Webcomics

• The native Qen of the planet Paa'Q in Drive are amphibious whale people and until a devastating pandemic drove the few survivors to take shelter on dry land most of their civilization was underwater...including all their infrastructure for building and maintaining starships. The modern Qen are starting to rediscover their lost past and finding that a significant chunk of their old fleet survives perfectly intact on the ocean floor because everything was built with underwater operation in mind. This is very bad news for a human empire that thinks of them as easily suppressed primitives.

Web Original

• The xkcd blog What If? discusses the flying-inside-a-gas-giant version in "Jupiter Submarine". It turns out that even a proper submarine, designed to direct atmospheric pressure outward, would be crushed or melted by the time it got deep enough into Jupiter to float on its liquid "surface".
  To reach a depth where it could "float" in Jupiter, the submarine would have to go halfway to the center of the planet, where the intense pressure turns the air into a metallic soup that's hotter than the surface of the Sun. The pressure there would be so high that not only would the submarine be crushed, the substances that make it up would probably be converted into new and exciting forms.

Western Animation

• Futurama: Downplayed in "The Deep South". After the Planet Express ship is pulled to the bottom of the Atlantic Ocean, it can't function underwater and requires the crew heavily modifying it (specifically building a giant fish tail) for them to swim back to the surface. Despite this, the ship overall holds together, with all life support functions continuing to work with no problems, even in spite of the Professor pointing out that as a spaceship it wasn't designed to withstand the crushing atmospheric pressures.

Real Life

• NASA astronauts often practice extravehicular activities (EVAs) in their spacesuits underwater, which is called neutral buoyancy simulation. Obviously, this is done in shallow water so the pressure doesn't become a safety issue.

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