The Oera Linda Book is an ancient manuscript that has been shrouded in mystery for centuries. Written in an unknown language, the book is believed to be the only surviving record of a lost civilization that existed in the Netherlands during the Middle Ages.
From Wikipedia
The Oera Linda Book is a manuscript written in imitated Old Frisian, purporting to cover historical, mythological, and religious themes of remote antiquity, from 2194 BCE to 803 CE. Among academics in Germanic philology, the document is considered to be a hoax or forgery.