In 1086 William the Conqueror completed a comprehensive survey of England and Wales. “The Domesday Book”, as it came to be called, contained details of 13,418 places and 112 boroughs—and is still available for public inspection at the National Archives in London. Not so the original version of a new survey that was commissioned for the 900th anniversary of “The Domesday Book”. It was recorded on special 12-inch laser discs. Their format is now obsolete.

The digital era brought with it the promise of indefinite memory. Increased computing power and disk space combined with decreasing costs were supposed to make anything born digital possible to store for ever. But digital data often has a surprisingly short life. “If we’re not careful, we will know more about the beginning of the 20th century than the beginning of the 21st century,” says Adam Farquhar, who is in charge the British Library’s digital-preservation efforts.