Rongorongo is probably a language, apparently scholars aren't sure. It's a system of glyphs that appears totally unrelated to any other system of writing, and has been found only on Easter Island, aka Rapa Nui. The name rongorongo comes from the Rapanui word meaning chant out. This is short for kohau motu mo rongorongo, or "lines incised for chanting out."
There's not a lot of wood on the island, so this stuff was inscribed on whatever bits writers could get their hands on. Inscriptions were made with sharks' teeth and other implements.
All of the samples that exist can be traced back at most a couple hundred years - the 1600s at the earliest, but by the 20th century, no one was left on the islands who could read or translate the glyphs. That no one was left isn't surprising - there were crazy wars and plagues going on in Easter Island Land back in the day, and there weren't many folks there to begin with.
By the time folks from off the island took an interest, most of the artifacts that contained the glyphs had been destroyed - since nobody could read them anymore, folks were using them as firewood, as spools for fishing line, and other stuff like that.
Reading about this got me thinking about Easter Island too. The place is teeny tiny, with a population to match. It's also super-remote. But there's Rongorongo, and then of course, there are the Moai. The Moai, also known as the Easter Island Heads, are thousands of years old, and what blows my mind about them is that nobody knows why a couple thousand people on a tiny remote island decided to start building them, or why they stopped. And now they're just out there - this singular, unique phenomenon, just floating out there in the world. Wild.
Info and photos from http://www.netaxs.com/~trance/fischer.html, Wikipedia, and The World in Words.