In Wakayama, Japan you will find the world's deepest mailbox at 10 meters under water. The town of Susami needed more tourism and decided on this unusual idea.

The underwater mailbox receives up to 1,500 letters per day, waterproof letters that are sold in town. Then people dive to post their mail, which is collected periodically.

While this is the deepest mailbox according to Guiness Book of World Records, it is worth to mention that various other countries have submarine mailboxes as well...