1693 VANUATU (Mele Island) - The Underwater Post Office

"Vanuatu Post has created an official Post Office with a difference. It is the world’s only Underwater Post Office.. Situated within the Hideaway Island marine sanctuary, the Post Office sits in around three metres of water and both divers and snorkellers are able to post special 'waterproof postcards'. If snorkellers cannot duck-dive down that far, Hideaway Island staff will be on the spot to help out. During opening hours the cards will be cancelled/embossed by the postal staff in the Underwater Post Office. Instead of being stamped with ink to show that the card has been sent, the Post office has developed a new embossed cancellation device."

1344 VANUATU (Tanna Island) - Yasur Volcano

"Opened in May 2005, the postal outlet on Tanna Island's Mt Yasur is recognised as the only post box in the world on a live volcano. Known as Volcano Post, this follows the famous Underwater Post Office in the second of Vanuatu Post's Xtreme series of unique and innovative postal facilities where official mail is collected, specially postmarked and sent throughout the world. The incredible postal locations enable visitors to mail home memories of Vanuatu's breathtaking sights and adventures. [...] All mail receives a unique Volcano Post cancellation, which will become sought-after items amongst the world's stamp collectors. [...] Imagine showing your friends a postcard from  the Volcano Post Box, which was sent to you just a lava rock's throw from molten lava with temperatures over 1000°C!" Between you and me, if they strove to open a post office on a volcano, they could apply the postmarks in a correct way, not randomly and incomplete.