The Postmaster of Susami, Japan had a rather interesting idea in 1999. In order to attract more tourism to the town, he decided to take an old post office box and place it under water. While this may sound like a rather peculiar ploy to bring people into the city, it worked.
The post office box placed 10 meters underwater just off the coast is actually functional. Every day, the mail is collected and taken to the post office so that it may continue onward to its destination. At its busiest, this single underwater post box gets up to 200 pieces of mail per day.
Now, divers come from all over the world for the chance to mail a letter from underground. The video below will take you even deeper into this interesting story.