In July 1562, at Maní in the Yucatán, a Franciscan named Diego de Landa held an auto-da-fé and burned the written record of the Maya.
He was explicit about why. The books contained, as he put it, nothing but superstition and the devil's falsehoods. He burned them anyway knowing the Maya valued them, and he noted with some satisfaction that it caused them enormous grief.
Four codices survive. Four. The Dresden, the Madrid, the Paris, and the disputed Grolier, preserved by the accident of being somewhere his soldiers did not reach.
Out of what was probably thousands.
And most of what we know of Maya astronomy comes from one of them. The Dresden Codex contains Venus tables tracking the synodic period to an accuracy of about two hours over five centuries.
That is one book. We have no idea what was in the others, because there are no others.
This was not collateral damage in a war. Maní was a scheduled event, conducted by an educated man, in peacetime, against a specific body of writing, on the grounds that it should not exist.
The irony is thick enough to be unbearable. De Landa is also the reason we can read Maya glyphs at all. He compiled an account of Maya customs, and in it recorded what he thought was an alphabet, dictated to him by a Maya informant who was apparently answering a slightly different question than the one being asked. It is not an alphabet, it is a partial syllabary, and it was wrong in ways that misled scholars for a century. But it was the key. Yuri Knorozov used it in the 1950s to begin the decipherment.
So the man who destroyed the library also, accidentally and incompetently, preserved the means of reading what he had destroyed. He left us the key to a door he had already bricked up.
When the chain breaks through neglect, as at Alexandria, the knowledge can sometimes be recovered: a copy turns up, a tradition preserved it elsewhere, a later century retranslates it. When the register is falsified, as with Great Zimbabwe, the evidence survives and the record can be corrected.
Maní is neither. The material is gone, the tradition that produced it was destroyed along with it, and there is no second copy anywhere on earth because the Maya were the only people who had it.
We cannot even say what we lost. That is the defining feature of it.
He thought he was burning superstition.
He was burning the only copy.