Arcologies are large, self-sustaining cities with extremely high human population densities. They are usually presented as tall structures that take advantage of height to minimize humanity's footprint on the environment. Arcologies are found frequently in science fiction and games.
No true arcology has yet been built, although there is an experimental town in Arizona that is attempting an arcology-like model. A true arcology would contain a complete internal ecosystem, or even function if sealed airtight. For this reason, arcologies have been proposed as a paradigm for the colonization of other planets.
The term was invented in the 50s by Italian-American architect Paolo Soleri, who blended together the words architecture and ecology. The arcology concept is meant to minimize the necessary structure for high human population densities by ensuring everyone has direct access to them. Arcologies are also meant as an alternative to slums, where the population density outweighs the intended carrying capacity of the civil infrastructure. The first arcology to be described in fiction was probably "The Last Redoubt" from The Night Land by William Hope Hodgson, first published in 1912. In modern times, the arcology has been featured as a futuristic building in the popular computer game SimCity 2000, and the movie Blade Runner.
An arcology must carefully balance human housing and resource needs with the natural needs of an artificial ecology, all sealed within a secure infrastructure. The ecology must be well-balanced, with no species going extinct or reproducing beyond control. Human waste must not cause the deterioration of the ecology. Because achieving this fine-tuned balance is not easy, and has not yet been done, design of an arcology needs input from civil planners, biologists, architects, naturalists, the public, and more.
Arcologies might be thought of as a social experiment. Collective social resources like libraries and hospitals could be exploited more thoroughly by the members of the populace. Private transport would be sacrificed in favor of public transport systems. Arcologies have been designed by a number of architects in different countries, but none of these designs has yet been implemented. The Japanese government has announced several long-term projects involving the possible construction of arcologies, including plans for a 300-story arcology called X-Seed 3000.