Mark Irons holds out a damaged music CD in Corvallis, Ore., Monday, May 3, 2004. 'CD rot' is a gradual deterioration of the data-carrying layer--it's not known for sure how common the blight is but it's just one of a number of reasons that optical discs may be a lot less long-lived than first thought.
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http://www.rdrop.com/~half/General/CDRot/CDRot.html