Mr. Matsumoto had an idea.

That idea would put his small town of Susami on the map.

(orchestral violin music)

(bouncy piano music)

This is Mr. Matsumoto.

He ran the post office in Susami.

I worked at the post office for about 40 years.

I came up with this idea to make an underwater mailbox.

I thought if would be interesting if divers could send letters from under the sea.

- [Narrator] In a classic, if you build it,

they will come story, they built it and people came.

Almost 38,000 postcards have been mailed from the underwater mailbox so far.

- [Narrator] This is Mr. Nakanishi.

He runs a dive shop here in Susami

and he manages the postbox.

The underwater mailbox is at a depth of about 10 metres (32 feet).

It's in the Guinness Book of World Records because it is the deepest mailbox in the world.

- [Narrator] Since the box is underwater,

you can't just mail a typical postcard.

So it goes like this, you come into Mr. Nakanishi's

dive shop, buy a postcard, write your note.

People use water-resistant postcards.

They write on the postcards with oil-based markers.

- [Narrator] Gear up, dive down, slip the note

into the red mailbox sitting on the ocean floor,

and according to Mr. Nakanishi, it will actually get

to its destination.

I dive down to the underwater mailbox everyday to collect the postcards.

After I collect them, I deliver them to the local post office.

I think the underwater mailbox has been a real asset for tourism in Susami.

People come here from far away to mail postcards from it.

- [Narrator] Ultimately, Mr. Matsumoto's idea worked.

The mailbox made Susami a destination for divers.

It made the town special.

I cam up with the idea to make Susami more appealing.

There's no other place where you can do this, this makes it special.

(bell chiming)