Kryptos
A copper sculpture outside CIA headquarters carrying four encrypted passages, the last of which has held out since 1990.
The artist Jim Sanborn unveiled the piece in November 1990 at Langley, Virginia. The first three passages, known as K1 to K3, were solved publicly by Jim Gillogly in 1999, although analysts at the CIA and NSA had already cracked them in-house. K1 and K2 are Vigenère ciphers over a keyword-mixed alphabet, K3 is a transposition cipher, and deliberate misspellings in the text kept solvers guessing for years.
K4 is 97 characters long and still has no public solution. Sanborn has released clues in instalments: BERLIN, CLOCK, NORTHEAST and EAST sit at known positions in the plaintext. That a world of amateurs and an intelligence agency haven't cracked four lines of sculpture says a lot about how little text it takes to make frequency analysis useless.