The Library of Alexandria
The library makes it on the list as it was rumoured to have been the repository of a collection of over 1 million scrolls, estimated to be all the written works in the world at the time. Founded in 300 BC, many scholars came to study, and the legends tell that anyone who visited the library, had to relinquish any books on their person, so that it could be copied and added to the collection.
As to the destruction of the library, there are many competing theories dating back to the 1st or 2nd century BC, stating that it could have been Julius Caesar who accidentally set fire to the library, or an invasion that caused the library to be sacked and burned. In any case, it’s safe to say that what that building contained was priceless and who knows what knowledge was lost in that fire.