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 atmosphere.
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 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 Venerean 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 Venerean 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 Venerean 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.
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).
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.
The " War of Powers" series by Robert E. Vardeman and Victor Milan features a Sky City ruled by a race of human overlords called the Sky born who conquered the city from its original reptilian inhabitants. The city is powered by dark magic and floats in a set pattern over 5 surface cities
Film and television
Cloud City on the planet Bespin, in the Star Wars film The Empire Strikes Back.
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 SG-1 have floating cities.
Metro City in the film Astro Boy 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 Laputa: Castle in the Sky involves a floating city hidden in the clouds called "Laputa," a name borrowed from Swift's Gulliver's Travels.
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 2011 animated TV series ThunderCats, a race of Birdmen live above the clouds in a technological floating city named Avista, powered and suspended in mid-air by the Tech Stone, one of Mumm-Ra's four Power Stones.
The animated television show My Little Pony: Friendship Is Magic depicts a city made entirely of clouds called Cloudsdale, a pun on the Clydesdale horse breed.
The sky borne metropolis of Caldoria in The Journeyman Project and its remake Pegasus Prime.
In Minecraft, there is a floating island called the End, populated by gangly ominous-looking humanoid creatures called Endermen.
Tales of Symphonia features a floating city named Exire, home to the outcast Half-Elves.
Chrono Trigger features the Kingdom of Zeal, a floating island based magical kingdom, encountered in 12000 BC.
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.
Metroid Prime 3: Corruption features SkyTown, a floating city on planet Elysia.
Mother 3 features a floating city named New Pork City which also appears in Super Smash Bros. Brawl.
There are several floating cities in the games Skies of Arcadia and Skies of Arcadia Legends.
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.
Sonic the Hedgehog 3 features Angel Island, a floating island.
Vane, the city of magicians of Lunar: Silver Star Harmony.
Final Fantasy features the Floating Castle, a floating city that resembles a space station.
Final Fantasy V features a floating city named the Ronka Ruins.
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.
The RPG Xenogears features a floating city-airship named Solaris.
Skyborn is a commercial RPG Maker game which it features a floating city.
Heart Forth, Alicia is an upcoming indie RPG game for the PC which it features a floating island, Auster.
Xenoblade Chronicles is a science fiction role-playing video game for the Nintendo Wii which it features a floating city, Alcamoth.
Tokyo Wakusei Planetokio is a science fiction adventure video game for the Sony PlayStation where the main action takes place in floating cities.
Kimino Yusha is a science fiction role-playing video game for the Nintendo DS which it features a floating land, Midalias.
In RuneScape, each clan is assigned to a floating island built by Armadyl, called a "Clan Fortress".
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.
