A megastructure, a concept which first entered into architectural discussion in the late 1950s and 1960s, has an evolving definition. The basic, modern definition is that a megastructure is simply an exceptionally large, man-made, self-supporting structure. Although true city-sized megastructures are still only found in science fiction, a number of existing structures can be counted under this definition, including the Great Wall of China and some extremely large skyscrapers.
In 1968, architect Ralph Wilcoxon defined a megastructure as a grouping of modular units which could be built upon and expanded nearly indefinitely. Smaller, prefabricated units could be added within the overarching megastructure, suited to the specific needs of its occupants. The megastructure's adaptability would enable the "hardware" of everyday life, such as utilities, to be run through it in easily accessible conduits.
Some architects, such as Reyner Banham, saw the megastructure as a way to combine the vision of city planners and architects. His book Megastructure: Urban Futures of the Recent Past was the essential guide to the movement. Like-thinking planners and architects believed that planning should be viewed on a much larger scale, and megastructures could provide real solutions to sprawl and disorganized, inefficient cities.
Today, megastructures follow less utopian ideals, and are built on a smaller scale than some of the earlier megastructure proposals. Nevertheless, a megastructure is typically a large, man-made, self-supporting structure or building. The definition often includes structures made of many smaller structures grouped together, a city housed within a single structure, or a bridge. There are no definitive requirements for a structure to be deemed a megastructure; just that it is unique in its size and engineering.
Due to the increasing number of megastructures being built, public interest has prompted television programs that explore these awe-inspiring structures. Among the megastructures profiled are the U.S.S. Ronald Reagan, the Sears Tower, the English Channel Tunnel (or "Chunnel"), and the Akashi Kaikyo Bridge in Japan. The Great Wall of China is also considered a megastructure, stretching nearly 4,000 miles (6,352 km) long.
The U.S. Ronald Reagan, which has been called a "floating city," can accommodate room for three months of supplies, and serve over 18,000 meals per day. The Chunnel is 32 miles (50 km) long — with 25 (40 km) of them under water. The Sears Tower is another famous megastructure, standing tall at 1,454 feet (443 m).
While the human imagination is only limited by current engineering and construction technology, fantasy megastructures abound in literature, gaming and film. The Death Star in the Star Wars series of films is a well known fictional megastructure, and many more are found in games like Halo and various science fiction stories. Many conceptual megastructures, such as space elevators and space stations, may someday become reality.