A megastructure, in science fiction an speculative (or exploratory) engineering, is an enormous self-supporting artificial construct. The definition is often informal and varies from source to source, but generally requires at least one dimension to be in the hundreds of kilometers. 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 of megascale engineering or astroengineering are megastructures.