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.