This is a great story, it sounds like the start of an Umberto Eco novel...
A medieval prayer book has yielded three key ancient text buried within its parchment. Works by mathematician Archimedes and the politician Hyperides had already been found buried within the book, known as the Archimedes Palimpsest. But now advanced imaging technology has revealed a third text - a commentary on the philosopher Aristotle.
One of the recycled books was proving extremely difficult to read. The book was being read using a technique called multispectral imaging which uses photographs taken at different wavelengths to enhance particular characteristics of the imaged area. Subtle adjustments of this method suddenly enabled other hidden words to be revealed. "Even though I couldn't read Ancient Greek, just the fact that I could see the words gave me shivers," said Professor Roger Easton
The passage has been identified as a relatively early commentary to Aristotle's Categories, Professor Roger Easton. Project director William Noel added, “At this point you start thinking striking one palimpsest is gold, and striking two is utterly astonishing."
link from BBC.
One could interpret this is a mode of modern condescension, condemning the medieval scribes that erased these classic manuscripts to write their own prayers. However I sympathize, having just given a way a load of books myself to make space on my groaning bookshelves, I am very well aware that pragmatic considerations in this matter can outweigh more profound ones. And anyway, it seems the worst kind of cultural relativism to valorise one knowledge episteme and condemn those who do not agree with the opinion.