This is an incredibly fascinating topic to me, first uncovered during a liberal arts course at RISD some years ago: how do you tell civilizations 10,000 years from now to avoid nuclear waste? Juliet Lapidos elaborates more on the topic for the Slate. According to the article, the oldest languages we have are from the Chinese, dating back some 5,000 years. She doesn't mention the prehistoric caves of Lascaux in France, which at 16,000 years old, contain symbols we can recognize. The bulls, bison, and cattle still make sense - they are from our environment - but there are other marks which have not yet been deciphered. They mean something, but the signs are unrecognizable. What signs can be created today that will outlive languages and generations? It's a big question, and the answer is probably in the one continuous vehicle that outlives us; nature.