Megastructures are very large man made objects, ranging from ziggurats, to skyscrapers, to hypothetical, star-sized artificial constructs. One requirement of a megastructure is that it is self-supporting; other criteria such as rigidity or contiguousness are sometimes also applied (so large clusters of associated smaller structures may or may not qualify). Megastructures are the products of megascale engineering (building things larger than 1000 km, e.g. the Great Wall of China) or astroengineering (building in outer space, e.g. the International Space Station).
Most megastructure designs could not be constructed with today’s level of industrial technology. This makes their design examples of speculative (or exploratory) engineering. Those that could be constructed easily qualify as megaprojects (construction projects in the billion dollar range).
Megastructures are also an architectural concept popularized in the 1960s where a city could be encased in a single building, or a relatively small number of buildings interconnected. Such arcology concepts (enormous habitats of extremely high population density) are sometimes called hyperstructures and are popular in science fiction. Megastructures often play a part in the plot or setting of science fiction movies and books, such as ‘Rendezvous with Rama’ by Arthur C. Clarke.
In 1968, architect Ralph Wilcoxon defined a megastructure as any structural framework into which rooms, houses, or other small buildings can later be installed, uninstalled, and replaced; and which is capable of ‘unlimited’ extension. Many architects have designed such megastructures. Some of the more notable such architects and architectural groups include the Metabolist Movement, Archigram, Cedric Price, Frei Otto, Constant Nieuwenhuys, Yona Friedman, and Buckminster Fuller. Other sources define a megastructure as ‘any development in which residential densities are able to support services and facilities essential for the development to become a self-contained community.’
Existing megastructures include: the Great Wall of China (a few meters wide and 6,352 km in length, about 4,160,000 m2), the Rice Terraces of the Philippine Cordilleras (a 10,360-square-kilometer sprawling agricultural landscape carved in the mountains by free tribesmen of Ifugao some 6,000 to 2,000 years ago), and the Large Hadron Collider (a ring 27 kilometers in circumference). The Expressways of China are the longest highway system in the world; networks of roads or railways, and collections of buildings (cities and associated suburbs), are usually not considered megastructures, despite frequently qualifying based on size, but an ecumenopolis (city planet) might qualify.
Proposed megastructures include: Atlantropa (a hydroelectric dam to be built across the Strait of Gibraltar, and the lowering of the surface of the Mediterranean Sea by as much as 200 meters), the Trans-Global Highway (highway systems that would link all six of the inhabited continents on Earth), and the Transatlantic tunnel (an underwater rail line spanning the Atlantic Ocean between North America and Europe).
Theoretical megastructures can be divided into Trans-orbital structures, Orbital structures, Planetary scale, and Stellar scale. The first group (also called Non-rocket spacelaunch) includes: Skyhooks (very tall structures that hang down from orbit), Space elevators (skyhooks that are fixed to the ground), Space fountains (a space elevator held in place kinetically, rather than by structural strength), and Lofstrom loops (‘Launch loops,’ 2000 km long iron loops projecting up in an arc to 80 km that are ridden by maglev cars while achieving orbital velocity). Moving up in size to orbital structures, proposed designs include: Orbital rings (enclosed loops slightly larger than the circumference of the Earth suspended in low earth orbit), and Bernal spheres (space colonies with a maximum diameter of 1.6 kilometers — the Stanford torus is a different design with a diameter just under 1.7 kilometers).
Planetary scale megastructures include: Orbitals (a ring-shaped space habitat orbiting a star; by giving a tilt to its orbit, there’s a convenient day and night experience on its surface), Globus Cassus (a proposed project for the transformation of Planet Earth into a much bigger, hollow, artificial world with the ecosphere on its inner surface), and Cloud nine (inventor Buckminster Fuller’s proposal for a tensegrity sphere of size a mile in radius which would be large enough so that it would float in the sky if heated by only one degree above ambient temperature, creating habitats for mini cities).
Most stellar scale Megastructure proposals are designs to make use of the energy from a sun-like star while possibly still providing gravity or other attributes that would make it attractive for an advanced civilization. Examples include: the Alderson discs (discs with an outer radius equivalent to the orbit of Mars or Jupiter and a thickness of several thousand miles; a civilization could live on either side, held by the gravity of the structure and still receive sunlight from a star bobbing up and down in the middle of the disc), the Dyson sphere (a structure or mass of orbiting objects that completely surrounds a star to make full use of its solar energy), Matrioshka brain (a dyson sphere converted into a massive computer), Stellar engines (uses the temperature difference between a star and interstellar space to extract energy or serves as a Shkadov thruster, which accelerates an entire star through space by selectively reflecting or absorbing light on one side of it), and Topopolis (also known as Cosmic Spaghetti, a large tube that rotates to provide artificial gravity).
The Daily Omnivore 
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