Amegastructureis abuilt structuretypically at least 1,000 kilometers in lengthin other words, at least 1megameter, hence the name. The definition is often informal and varies from source to source. By extension, some people apply the term to any especially large or tall building.Most megastructure designs could not be constructed with today's level of industrial technology. This makes their design examples ofspeculative (or exploratory) engineering. Those that could be constructed easily qualify asmegaprojects.Some sources define a megastructure as an enormous self-supporting artificial construct.Other criteria such as rigidity or contiguousness are sometimes also applied, so large clusters of associated smaller structures may or may not qualify.The products ofmegascale engineeringorastroengineeringare megastructures.Megastructuresare also anarchitecturalconcept popularized in the 1960s where a city could be encased in a single building, or a relatively small number of buildings interconnected. Sucharcologyconcepts are popular inscience fiction.

There are structures on Earth that may be considered megastructures, such as

http://www.aboutcivil.org/megastructures.html