size. However, an ecumenopolis might qualify. Proposed Atlantropa, a hydroelectric dam to be built across the Strait of Gibraltar, lowering the surface of the Mediterranean Sea by as much as 200 meters. Trans-Global Highway, highway systems that would link all six of the inhabited continents on Earth. The highway would network new and existing bridges and tunnels, not only improving ground transportation but also potentially offering a conduit for utility pipelines. Cloud nine is Buckminster Fuller's proposal for a tensegrity sphere 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 of thousands of people in each "Cloud Nine". Fuller also proposed a marine analog consisting of a hollow terraced floating tetrahedron of reinforced concrete measuring one mile from vertex to vertex supporting a population of one million living in air-deployed residential modules on the exterior with the requisite infrastructure providing utilities (water, power, sewerage, etc.) inside. The modules would have standardized utility ports so as to be completely livable within minutes of arrival, and could be subsequently detached and moved to other such cities. Theoretical A number of theoretical structures have been proposed which may be considered megastructures. Stellar scale 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. The Alderson disk is a theoretical structure in the shape of a disk, whose outer radius is equivalent to the orbit of Mars or Jupiter and whose thickness is several thousand miles. A civilization could live on either side, held by the gravity of the disk and still receive sunlight from a star bobbing up and down in the middle of the disk. A Dyson sphere (also known as a Dyson shell) refers to a structure or mass of orbiting objects that completely surrounds a star to make full use of its solar energy. A Matrioshka brain is a collection of multiple Concentric Dyson Spheres which make use of star's energy for computing. A Stellar engine either uses the temperature difference between a star and interstellar space to extract energy or serves as a Shkadov thruster. A Shkadov thruster accelerates an entire star through space by selectively reflecting or absorbing light on one side