Deepest postbox underwater

The world's deepest underwater postbox is located 10 m (32 ft 9 in) beneath the waters of Susami Bay, Japan. The postbox is opened only by authorised employees using post office keys. Special waterproof plastic postcards are used by the divers. Sports divers can post mail in the box, which is officially part of the Susami post office and opened daily with a special key. More than 4273 undersea mails have been collected in the postbox's first year of operation.