The worldâ€™s deepest underwater Postbox is located in Susami Bay in Japan, and is said to lie at a depth of 10 meters underwater. People usually travel from different cities to mail their letters to their family members located in other different cities. This postbox mainly lures those people who purchase the water-proof cards on which the message is written with the help of the oil-based paint markers, because of which the message can remain intact on the card despite the underwater immersion.
People then dive into the sea to post their mail, and it is said to be a thrilling experience for many people who engage in this activity. The underwater postbox actually came into being during a fair in the year 1999, as part of an effort to give special attention to Susami Bay. This idea was proposed by a then-postmaster who was 70 years old named Toshihiko Matsumoto.