A palimpsest is an ancient manuscript in which one text has been erased and then written over with another text. Some of our most famous ancient texts survived only in this way, including works by Archimedes, Cicero, Seneca, and Plautus, the oldest surviving textbook of Roman Law, and the oldest version of the Gospels in Syriac. But what, you have perhaps wondered, does a palimpsest actually look like? And how hard are they to read?
One palimpsest discovered in fairly recent times is a work by the ancient Greek doctor Galen, written over with a set of Syriac hymns. Scholars have been working on this for years, mapping out the text character by character and trying to fit it back together. Detailed photographs of the whole thing are online, at the Digital Galen Palimpsest. You can see from these that the older writing is quite faint and not at all easy to ready, which is why reading one of these texts takes thousands of person-hours of labor.