While Wakayama prefecture is most famous for its pilgrimage trail, holy mountains, and dramatic coastlines, one of the prefecture’s other claims to fame is the deepest underwater postbox in the world.
Located off the coast of the town of Susami, Wakayama, the postbox stands 10 meters (32.8 ft) underwater. It was placed there in 1999 by the Marine Sports Festival Committee and won the Guinness World Record in 2002. Divers sent over 30,000 pieces of waterproof mail from the box in 2011 alone.
To read more in the original Japanese and for photos, see 「日本一深いところにあるポスト 海中郵便ポスト」 on the Wakayama Prefectural Tourism website.