Web archiving aims preserve selected sets of internet pages. Because the pages themselves are inherently impermanent -- subject to degradation, obsolescence, malicious destruction, and inadvertant alteration or deletion -- they are preserved by replication and by migration from outdated to current-day formats. The replicated pages are kept in entirety -- with accompanying format files, graphic images, and the like -- on preservation servers in safe environments. Because no server or server environment can be made fully safe and because server storage is not costly, mirror sites are often used in widely separated geographic locations.