Hextonic drift is a phenomenon by which hexes change terrain type when not observed by talking units for very long periods of time. As a result, ancient maps of places that haven't received visitors for a very long time are generally inaccurate.
They never change all at once, and gradually become different terrain types over phases.
Some believe it to be a form of the obscure branch of magic known as deletionism.
Real World References
See the continental drift wikipedia article.