In 1258 Mongols under the command of Hulagu Khan sacked the city of Baghdad, destroying the House of Wisdom (the leading library in the leading intellectual center of the Arab world at the time).
It was a loss of Library of Alexandria proportions, to put it mildly.
In just one week, five hundred years’ worth of closely cared for texts were gone, either burned or dumped into the Tigris River.
Some survivors of the event claimed the Tigris ran red with blood, others that it ran black with ink. In truth, it was probably a bit of both.
One man claimed so many books were thrown into the river that they formed a bridge strong and wide enough for a man on horseback to cross (Harris).
^ “Conquest of Baghdad by the Mongols 1258”, Rashid-ad-Din's Gami' at-tawarih, circa 1300-1325. Watercolours and gold on paper. See here for a larger, more detailed image and more information on the art piece.
Harris, Michael H. History of libraries of the western world. Scarecrow Press, 1999.