An architectural structure of gigantic proportions, usually of the 20th Century and fantastical in nature. It is designed to contain all the requirements for human existence in one monumental building, effectively forming artificial land, as in the high-density projected "Arcologies" by architect Paolo Soleri for up to six million inhabitants. Also described as a city in one building. Although currently purely a fictional concept (usually science fiction) the idea of a megastructure may well be required if human beings ever find it necessary to colonise worlds other than this one.