Susami is a fishing town with a population of around 5,000, and is in Guinness World Records for having the deepest underwater postbox, at a depth of 10m (30 feet) off its coast.
Officially recognized as a mail collection point of the local postal system, 1,000 to 1,500 pieces of mail are dropped into the box each year. The postbox is for use by divers who buy water-resistant postcards at Yamatani's store and write messages on them with an oil-based paint marker. An employee of the shop collects the cards once every few days and takes them to the local post office. The items are then delivered to recipients within a week of being posted.
The postbox was created as part of a fair in 1999 to promote the Kumano Kodo pilgrimage trail and surrounding areas. With Susami having no special attraction, Toshihiko Matsumoto, 70, then-postmaster of the town, proposed an undersea postbox.
An old, red postbox was altered for the underwater collection of cards and installed when the fair began in April 1999. Some 32,000 items of mail have since been posted in the box, which was recognized by Guinness World Records as the "Deepest Underwater Postbox" in 2002.
As seawater severely corrodes the cast-iron box, two of them are used in rotation every six months, allowing each to be cleaned and repainted. "We have no replacement if they become unusable," Yamatani said.
Here's a short video that explains the story...