the Burning of the LIbrary of Alexandria was the most serious historical and cultural loss at that time on earth

Arguably, this library is most famous for having been burned down resulting in the loss of many scrolls and books; its destruction has become a symbol for the loss of cultural knowledge. Sources differ on who was responsible for its destruction and when it occurred. The library may in truth have suffered several fires over many years. In addition to fires, at least one earthquake damaged the city and the library during this time. ^[1] Possible occasions for the partial or complete destruction of the Library of Alexandria include a fire set by the army of Julius Caesar in 48 BC and an attack by Aurelian in the 270s AD.

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Library of Alexandria - Wikipedia

The time this burned down the world was in turmoil and in some ways it has always been. So, when cultural losses occur on this level they simply cannot be replaced. What was really lost there? It is unknown beyond saying:

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The library was created by Ptolemy I Soter, who was a Macedonian general and the successor of Alexander the Great.^[3]Most of the books were kept as papyrus scrolls. It is unknown precisely how many such scrolls were housed at any given time, but estimates range from 40,000 to 400,000 at its height.

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So we are talking about up to 400,000 books and records that were irreplaceable to humanity at that time regarding incredible events of world history, technology, religion, philosophy and Science. So, this was a devastating loss to mankind at the time.

I also believe the loss of Nalanda University in India was a similar devastating loss to all Asian cultures as well when that happened.