In science fiction, floating cities are settlements that strictly use buoyancy to remain in the atmosphere of a planet. However the term generally refers to any city that is flying, hovering, or otherwise suspended in the air via any means technological or magical.
In Jonathan Swift's satirical novel Gulliver's Travels, Swift envisioned Laputa, an island city that floated in the sky. The island was suggested to levitate above the Earth by use of the force of magnetism. In the 1920s, Hugo Gernsback speculated about floating cities of the future, suggesting that 10,000 years hence "the city the size of New York will float several miles above the surface of the earth, where the air is cleaner and purer and free from disease carrying bacteria." To stay in the air, "four gigantic generators will shoot earthward electric rays which by reaction with the earth produce the force to keep the city aloft."
Although Swift's proposal was intended as satire, Buckminster Fuller proposed the concept more seriously in the form of the Cloud nine (Tensegrity sphere) megastructure, in which he envisioned structural spheres that float freely in the sky, allowing passengers a migratory lifestyle and a solution to the depletion of Earth's resources. He proposed a 1-mile-diameter (1.6 km) geodesic sphere that would be heated by sunlight, functioning as a thermal airship.
A design similar to Fuller's Cloud Nine might permit habitation in the upper atmosphere of Venus, where at ground level the temperature is too high and the atmospheric pressure too great. As proposed by Geoffrey A. Landis, the easiest planet (other than Earth) to place floating cities at this point would appear to be Venus. Because the thick carbon dioxide atmosphere is 50% denser than air, breathable air (21:79 Oxygen-Nitrogen mixture) is a lifting gas in the dense Venusian atmosphere, with over 60% of the lifting power that helium has on Earth. In effect, a balloon full of human-breathable air would sustain itself and extra weight (such as a colony) in midair. This means that any large structure filled with air would float on the carbon dioxide, with the air's natural buoyancy counteracting the weight of the structure itself.
At an altitude of 50 km above the Venusian surface, the environment is the "most Earthlike in the solar system", with a pressure of approximately 1 bar and temperatures in the 0°C-50°C range. Because there is not a significant pressure difference between the inside and the outside of the breathable-air balloon, any rips or tears would cause gases to diffuse at normal atmospheric mixing rates, giving time to repair any such damage. In addition, humans would not require pressurized suits when outside, merely air to breathe and a protection from the acidic rain.
Since such colonies would be viable in current Venusian conditions, this allows a dynamic approach to colonization instead of requiring extensive terraforming measures in advance. The main challenge would be using a substance resistant to sulfuric acid to serve as the structure's outer layer; ceramics or metal sulfates could possibly serve in this role. (The sulfuric acid itself may prove to be the main motivation for creating the structure in the first place, as the acid has proven to be extremely useful for many different purposes.)
Other planets 
In addition to Venus, floating cities have been proposed in science fiction on several other planets. For example, floating cities might also permit settlement of the outer three gas giants, as the gas giants lack solid surfaces. Jupiter is not promising for habitation due to its high gravity, escape velocity and radiation, but the solar system's other gas giants (Saturn, Uranus, and Neptune) may be more practical. In 1978, the British Interplanetary Society's Project Daedalus envisioned floating factories in the atmospheres of Jupiter refining Helium-3 to produce fuel for an interstellar probe. Michael McCollum notes that the "surface" gravity of Saturn (that is, at the visible cloud layer, where the atmospheric pressure is about the same as Earth's) is very close to that of Earth, and in his novel The Clouds of Saturn, he envisioned cities floating in the Saturnian atmosphere, where the buoyancy is provided by envelopes of hydrogen heated by fusion reactors. Uranus and Neptune also have "surface" gravities comparable to Earth's, and even lower escape velocities than Saturn. Cecelia Holland populated Jupiter, Saturn and Uranus with mutant humans, the Styth, in floating cities in her only SF novel, Floating Worlds (1975).
Fictional examples 
||This section needs additional citations for verification. (December 2007)
- The novel Orion Shall Rise by Poul Anderson features an aerostat city called Skyholm, located above present-day France.
- In the novel The Ringworld Engineers, Louis Wu seeks a way to save the Ringworld by bartering for information in the library of a floating city.
- Airhaven is a floating city in the Mortal Engines Quartet, that, through attaching gas bags, lifted itself into the air to avoid cities trying to eat it according to Municipal Darwinism.
- There are numerous floating habitats on the Venus-like planet Chilo in Tobias S. Buckell's novel Sly Mongoose. Buckell credits Geoffrey A. Landis with providing the background information on the floating cities.
- Saga of Seven Suns by Kevin J. Anderson has giant, manned, gas mining platforms, that mine the hydrogen necessary to, among other things, distill into ekti, a vital stardrive fuel.
- Charles Stross's novel Saturn's Children begins in a floating city on Venus.
- In Hannu Rajaniemi's novel The Quantum Thief, the Mars colony began as a slave-labor latifundia. After war developed, all entities began taking turns being the beings who kept the city rolling (and deflecting the permanent attack vectors that had been created during the war). The city kept rolling, with everyone's help.
- The Cities in Flight series by James Blish propose a universe in which cities cast adrift from the Earth, powered by a fictional spindizzy drive.
Film and television 
- Cloud City on the planet Bespin, in the Star Wars universe.
- Stratos, on the planet Ardana, in Star Trek episode "The Cloud Minders".
- In Firefly episode "Trash", the planet Bellerophon is the site of dozens of floating estates with "gracious living, ocean views and state-of-the-art security."
- Atlantis from the Stargate universe is a probable example of a floating city. Although the city ship weighs several million tons, it is buoyant enough to float on water and, given that its energy shield can hold the atmosphere inside nearly indefinitely, it should be able to float in a particularly dense atmosphere.
- The Nox of Stargate SG1 have floating cities.
- Metro City in the Astro Boy (2009 film) is floating above the surface.
- In the film Steamboy, a "Steam Castle" was shown, which was essentially a floating city, kept in the air by means of steam that was directed towards the soil
- Hayao Miyazaki's film Castle in the Sky involve floating structures similar to a floating city.
- In the anime and manga One Piece there are Sky Islands, cities built on a specific type of cloud that has hard, land-like properties, allowing civilizations to have ground to traverse and build on using the same cloud material, along with an ocean-like cloud throughout, making it a close parallel to a normal earthbound island. The unique environment of the Grand Line, an equatorial ocean that circles the globe and possesses all matter of mythical weather patterns, islands and equally mythical sea-behemoths, allows for these Sky Islands to occur regularly- yet are so rarely witnessed even the denizens of the legendary Grand-Line perceive them as a myth.
- In the game Minecraft, there is a floating island called the End, populated by gangly ominous-looking humanoid creatures called Endermen.
- The RPG Tales of Symphonia features a floating city named Exire, home to the outcast Half-Elves.
- Kingdom of Zeal, a floating island based magical kingdom, encountered in 12000 BC in Chrono Trigger.
- City in the Sky, the seventh dungeon level of The Legend of Zelda: Twilight Princess.
- Skyloft, the town in which The Legend of Zelda: Skyward Sword begins.
- SkyTown is a floating city on planet Elysia in Metroid Prime 3: Corruption from the Metroid video game series.
- New Pork City, the last stage in the 2006 videogame Mother 3.
- There are several floating cities in the games Skies of Arcadia for the Dreamcast and Skies of Arcadia Legends for the Nintendo GameCube.
- In World of Warcraft, Dalaran is a major city that floats above Crystalsong Forest in the center of Northrend.
- The setting of the Eclipse Phase role-playing game includes floating cities on Venus and Saturn.
- Columbia, the setting for the game BioShock Infinite.
- Angel Island is a floating island (Sonic 3 and Knuckles)
- Vane, the city of magicians of Lunar: Silver Star Harmony.
- Final Fantasy XII has several in the form of solid earth islands suspended in the air by mystically charged stones inherent to the earth making up the foundation of the floating city, with Bhujerba being the only one visited and seen firsthand in the game.
- Sanctuary is the floating hub city of Borderlands 2.
- Glitzville, from Paper Mario: The Thousand-Year Door, is a small city that floats above Rogueport and its surrounding areas.
- In the Evolo Competition 2012, Wei Zhao describes an island floating in the air using magnets, much akin to the hoverboard featured in the "Back to the Future"-films. According to Stef Wiegers of the High Field Magnet Laboratory however, the design could never be created as the earth's magnetic field is too weak to support such mass.
- The webcomic Dresden Codak includes the city of Nephilopolis, a city constructed in the wreckage of a giant floating ancient robot.
See also 
- ^ see illustration from the magazine Science and Invention, February 1922, on Urban Utopias web page (accessed September 23, 2008)
- ^ Fuller, Buckminster; Kiyoshi Kuromiya (1981). Critical Path. St. Martin's Griffin. ISBN 0-312-17491-8.
- ^ Landis, Geoffrey A. (Feb. 2-6 2003). "Colonization of Venus". Conference on Human Space Exploration, Space Technology & Applications International Forum, Albuquerque NM.
- ^ Atkinson, Nancy (July 16, 2008). "Colonizing Venus With Floating Cities". Universe Today. Retrieved 2008-09-23.
- ^ Landis, quoted in Atkinson op. cit.
- ^ Seif, A. (1985). "Thermal Structure of the Atmosphere of Venus". In Hunten et al.,, D. M. Venus. Tucson, AZ: University of Arizona Press. pp. 215–279.
- ^ Buckell, Tobias, "The Big Idea: Tobias Buckell" interview in Whatever, Aug. 19, 2008 (Retrieved on 2008-09-23).
- ^ Floating city design in Evolo 2012
- ^ Kijk magazine, 6/2012