Susami, Japan is one of the few places on earth that people travel to just to send a post card. But there’s a good reason the town’s postal service has been attracting tourists since 1999: That’s the year former postmaster Toshihiko Matsumoto came up with the idea of installing the world’s first underwater mailbox just offshore.

Great Big Story takes a deeper look at this unusual destination in a recent video. To use the mailbox, senders must purchase a waterproof postcard from the local dive shop and write with oil-based markers. Once their parcel is ready, they have to slip into their diving gear and make the trek to the post box 30 feet beneath the ocean surface. Every day the dive shop manager collects whatever cards have been left in the box and delivers them to the post office on land.

Matsumoto originally saw the mailbox as a way to bring tourists to his small town, and his plan paid off: Nearly 38,000 letters have been sent via the undersea system to date. (The postal service in Vanuatu expanded the concept and opened an undersea branch in 2003, complete with attendant.) To see the mailing process at work, check out the video below.

[h/t Great Big Story]