You’re probably familiar with the postal credo of not letting rain or snow or sleet interfere with duties. In the South Pacific Ocean nation of Vanuatu, that guarantee apparently extends to being totally submerged underwater.
As Lonely Planet notes, island postal officials debuted a deep-sea post office adjunct in 2003. Tourists to the collection of more than 80 islands can dive roughly 10 feet (about 3 meters) down near Hideaway Island to discover a staffed aquatic postal station. Waterproof postcards and stamps purchased on dry land can be mailed via the sea, with visitors alerted to the window being occupied by a flagged bob in the water. (If not, they can drop mail off in a separate slot.) Vanuatu clerks can even postmark the correspondence, substituting ink for an embossing device that proves it's in transit. The cards can then be sent internationally.
If the idea of conducting mail transactions while snorkeling isn’t extreme enough for you, Vanuatu also offers a drop-off box situated on an active volcano on Mt. Yasur.
[h/t Lonely Planet]