Contrary to popular belief, electronic data has proven to be much more ephemeral than books, journals or pieces of plastic art. After all, when was the last time you opened a WordPerfect file or tried to read an 8-inch floppy disk? "Even over the course of 10 years, you can have a rapid enough evolution in the ways people store digital information and the programs they use to access it that file formats can fall out of date," McDonough said. Magnetic tape, which stores most of the world's computer backups, can degrade within a decade. According to the National Archives Web site by the mid-1970s, only two machines could read the data from the 1960 U.S. Census: One was in Japan, the other in the Smithsonian Institution. Some of the data collected from NASA's 1976 Viking landing on Mars is unreadable and lost forever.