rationalskepticism.org

Alexandria is the most famous because it had this magnificent library as part of the museum, and the mystery of what happened to this library, allegedly burned by the Muslims in the eighth century on the grounds that you didn't need to know anything except what's in the Koran. And this is not true. The library had disappeared long before the eighth century and probably was the victim of the kinds of disorders that began in the third century, when we began the course, the kind of disruptions of local society, opportunities for plunder, neglect. And the Museum was actually closed by the emperor Caracalla in the third century.

Arguably, this library is most famous for having been burned down resulting in the loss of many scrolls and books; its destruction has become a symbol for the loss of cultural knowledge. Sources differ on who was responsible for its destruction and when it occurred. The library may in truth have suffered several fires over many years. Possible occasions for the partial or complete destruction of the Library of Alexandria include a fire set by the army of Julius Caesar in 48 BC and an attack by Aurelian in the 270s AD.
After the main library was destroyed, scholars used a "daughter library" in a temple known as the Serapeum, located in another part of the city. According to Socrates of Constantinople, Coptic Pope Theophilus destroyed the Serapeum in AD 391, although it is not certain what it contained or if it contained any significant fraction of the documents that were in the main library. The library may have finally been destroyed during the Muslim conquest of Egypt in (or after) AD 642.