A functioning red Japanese post box standing on the seabed at 10m in a sheltered bay. You buy an oil-based-ink pen and a…A functioning red Japanese post box standing on the seabed at 10m in a sheltered bay. You buy an oil-based-ink pen and a plastic postcard in the village, dive down and post it, and a diver from the local post office collects the mail. Guinness recognises it as the world's deepest underwater postbox. Installed in 1999 by the local postmaster to promote Susami during a diving festival and simply never removed, it has since handled over 40,000 letters. It is emptied regularly by a postal diver and the mail is franked with a special postmark. The most charming entry on this entire list. Anchorage: Anchor in Susami Bay; small fishing harbour nearby. Hazards: None in calm conditions. More