what library was burn to the ground that was said to set mankind back thousands of years?

Anonymous2015-02-09T20:36:36Z

You're probably thinking of the great library in Alexandria that was burned by Crusaders. It didn't set humankind that far back, but it was indeed a major - and pointless- loss.

Anonymous2015-02-09T20:07:20Z

The Library of Alexandria, which was located in the city of Alexandria in Egypt.

staisil2015-02-09T20:27:07Z

Library of Alexandria

The library was created by Ptolemy I Soter, who was the successor of Alexander the Great.

Historians cannot agree on the cause of the fire.
http://www.ask-aladdin.com/Alex-Travel-Information/library_of_Alexandria.html