'World's deepest underwater postbox' is in Japan
A mailbox that is situated roughly 33 feet underwater in Susami Bay in Japan's Wakayama prefecture, is believed to be the world's deepest underwater postbox. The postbox, which is officially recognised as a mail collection point, receives up to 1,500 water-resistant postcards  each year. As seawater corrodes the box, two of them are used in rotation every six months.