Many institutions have their archives stored on CDs — but the discs aren't as stable as once thought. There is no average life span for a CD, says preservationist Michele Youket, "because there is no average disc." --- Back in the 1990s, historical societies, museums and symphonies across the country began transferring all kinds of information onto what was thought to be a very durable medium: the compact disc. Now, preservationists are worried that a lot of key information stored on CDs — from sound recordings to public records — is going to disappear. Some of those little silver discs...