The Lost Library of the Tsars
In the Library of the Moscow Tsars, an extensive collection of ancient Greek texts was said to be housed. A library building was constructed in the 16th century to protect the vast amount of texts. It was one of their own, however, that caused the library collection to be lost.
Ivan the Terrible removed most of the items from the Library of the Moscow Tsars and hid the ancient texts. The items have yet to be found.