The Great Library of Alexandria was one of the wonders of ancient civilisation having collected many thousands of scrolls containing knowledge and literature from across the known world.
The 2009 movie Agora is partially about its destruction and tells this story (my emphasis):
When the Christians start defiling the statues of the pagan gods, the pagans, including Orestes and Hypatia's father, ambush the Christians to squash their rising influence. However, in the ensuing battle, the pagans unexpectedly find themselves outnumbered by a large Christian mob. Hypatia's father is gravely injured and Hypatia and the pagans take refuge in the Library of the Serapeum. The Christian siege of the library ends when an envoy of the Roman Emperor declares that the pagans are pardoned, however the Christians shall be allowed to enter the library and do with it what they please. Hypatia and the pagans flee, trying to save the most important scrolls, before the Christians overtake the library and destroy its contents
Carl Sagan told a similar story in his series Cosmos (see this clip from about 3:30 in).
This version of the story has been frequently repeated as an argument about the irrationality of religion, but it has been disputed. For example, David Bently Hart (commenting on the movie) says the following:
The story he repeats is one that has been bruited about for a few centuries now, often by seemingly respectable historians. Its premise is that the Christians of late antiquity were a brutish horde of superstitious louts, who despised science and philosophy, and frequently acted to suppress both, and who also had a particularly low opinion of women.
Thus, supposedly, one tragic day in a.d. 391, the Christians of Alexandria destroyed the city’s Great Library, burning its scrolls, annihilating the accumulated learning of centuries, and effectively inaugurating the “Dark Ages.” ...
This is almost all utter nonsense, but I have to suppose that Amenábar [the director of Agora] believes it to be true.
The tale of a Christian destruction of the Great Library ”so often told, so perniciously persistent”is a tale about something that never happened .
So were mobs of Christians responsible for the destruction or is that just a modern myth?