Great Wonders (called "Wonders" in games where no distinctive adjective is necessary) are improvements that can be built by only one civilization. Wonders of the World are unique buildings built in cities. For each of them in the early Civilization games, and some in later games, only one "copy" of each wonder can exist. Wonders take a lot of time to build and are not always worth the effort. Many lose their effect (i.e. are made obsolete) as soon as any civ discovers a specified new technology.