When the Japanese town of Susami in the Nishimuro district of the Wayakama Prefecture, decided that they wanted to have more tourism, Toshihiko Matsumoto, the postmaster came up with the unique idea of putting a mailbox underwater. This mailbox currently in use, but it does require the purchase of waterproof cards and oil based pens. Once the postcard is written all the sender has to do is dive down and insert it into the mailbox at the ocean floor for later pickup. via Laughing Squid