"Vanuatu Post has created the world's only Underwater Post Office. Situated within a marine sanctuary, off the Island near Port Vila in Vanuatu, the Post Office can receive mail delivered in person if you are a visiting diver or snorkeller or sent via the Main Post Office to ensure all mail items receive the special cancellation from this unique underwater paradise.
What makes the postcard waterproof?
The postcards are made out of a synthetic product called Syntek and the weight is 340gms. So its plastic like product. The Syntec material is basically made of a mixture of clay (calcium carbonate) and polypropylene resin. It has the look, feel and printability of paper, plus the durability of plastic. They are waterproof, tear-resistant, impervious to grease, oils, chemicals and the elements. They can also withstand high temperatures and humidity."
Some of the stamps:
The World's Deepest Water-Filled Sinkhole - El Zacatón Cenote