The delightfully-named Rongo Rongo writings of Easter Island have been confusing historians and translation experts for centuries. They defy translation, don't appear to be connected to any root language, and there are no native speakers of it left on the island.
Discovered in 1864, it's thought that the wooden planks that contain the writing date back to around the 13th century. They clearly have some meaning, but it's likely we'll never find out what it is. It's like an ancient code!