Hey -- I'm guessing you may have seen this subthread on r/pics?

To be clear, the Library of Alexandria existed, as did the wonders commonly listed on the "Seven Wonders of the Ancient World" list (although there's a lot of dispute over what exactly the Hanging Gardens of Babylon were -- see this older thread on that topic). (Edited to clarify the lighthouse at Alexandria is on the Seven Wonders lists, not the library.)

The thing about the Library of Alexandria, though, is that its "burning" (it actually burned several times) gets kind of uncritically used as some sort of watershed event in the loss of knowledge from the ancient world, often by folks who are popular scientists but not great historians. There are a couple threads in our FAQ that talk about this -- while the library was a large and important one, knowledge that we have lost from the ancient world is mostly lost because it wasn't thought of as important enough to preserve (copy) over time.

If you have follow-ups about the Hanging Gardens, u/mythoplokos is the author linked to in that older thread, while u/xenophontheathenian answered about the Library of Alexandria.