Kryptos

This is totally the kind of thing that gives Dan Brown a half-lobster.

It's a big sculpture in the grounds of the CIA headquarters at Langley, and it contins a code that's never been fully broken. The bits that have been broken reveal mysterious fragments of conversations, some famous and others not-so-famous.

(Link to Wikipedia, although I probably originally learned about it from Darkroastedblend).

Turns out Dan Brown really does love it, so much so that he's put it (oh-so-inevitably) in his new book, "The Quest for more Movie Rights", or whatever.