A megastructure is a very large artificial object, although the limits of precisely how large vary considerably. Some apply the term to any especially large or tall building. Some sources define a megastructure as an enormous self-supporting artificial construct. The products of megascale engineering or astroengineering are megastructures. The lower bound of megastructural engineering might be considered any structure that has any single dimension 1 megameter (1000 km) in length. 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. 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 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, Ralph Wilcoxen 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. This type of framework allows the structure to adapt to the individual wishes of its residents, even as those wishes change with time. 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". 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. Existing There are structures that may be considered megastructures, such as The Great Wall of China is a human-built megastructure, a few meters wide and in length, about . The Rice Terraces of the Philippine Cordilleras, a sprawling agricultural landscape carved in the mountains by free tribesmen of Ifugao some 6,000 to 2,000 years ago. Skyscrapers represent the current state-of-the-art in large structure engineering. (See the List of tallest buildings and structures in the world and list of largest buildings in the world.) The Large Hadron Collider consists of, among other structures, a ring 27 kilometers in circumference. Networks of roads or railways, and collections of buildings (cities and associated suburbs), are usually not considered megastructures, despite frequently qualifying based on