Conversation so far:
when did the library of alexandria burn down
The Library of Alexandria, or part of its collection, was accidentally burned by Julius Caesar during his civil war in 48 BC, but it's unclear how much was actually destroyed.
what really happened to the library of alexandria if it wasn't completely burned down
Despite the widespread modern belief that the Library of Alexandria was burned once and cataclysmically destroyed, the Library actually declined gradually over the course of several centuries.

Question: when did the library start its decline
Self-contained rewrite: when did the library of alexandria start its decline