I just happened upon this rather interesting story. A group of researchers from Stanford University and the University of Manchester have recovered an aria from an 18th century Italian opera by x-raying the composer's manuscript. The opera, Medeé by Cherubini, was written in 1797 and after criticisms that it was too long Cherubini decided to blacken out the final aria.

The research team stated that the agent Cherubini used to blacken out the aria was non-ferrous which allowed them to uncover the iron ink beneath it. Now there is a complete legible score to the opera for the first time since it was originally written. I wonder if the full piece will be performed in its entirety now that it is possible.