In Susami Bay, Japan, a red postbox sits 10 meters below the sea. Divers can drop a waterproof postcard into it, where an authorized collector retrieves the mail for the ordinary postal system on shore.
Most mailboxes ask you to walk to the curb. Susami’s asks you to descend through clear seawater.
The unusual box is in Susami Bay, off the coast of Wakayama Prefecture in Japan. Guinness World Records’ record entry lists it as the world’s deepest underwater postbox, at 10 meters (32 feet 9 inches) beneath the water. The record page dates the entry to April 23, 1999, and says the box is officially part of the Susami post office.
The confirmed part: it accepts real mail
The mailbox is an active collection point on the seafloor. Guinness records that sports divers can post mail there using special waterproof plastic postcards. The same entry says authorized employees open the box with post-office keys. More than 4,273 pieces of undersea mail were collected during its first year of operation.
Susami Town’s own tourism guidebook describes the local “underwater post” as a landmark 10 meters below the sea and says that a special postcard deposited there can be sent around the world. The guidebook places the experience alongside guided diving and other marine activities, making the mailbox a piece of local infrastructure and a community attraction at the same time.
How underwater mail gets dry-land treatment
The strange part is the setting; the mail follows a familiar postal process. A diver carries the waterproof card down to the anchored box and places it inside. An authorized collector later opens the box, brings the cards back to shore and puts them into the normal delivery stream. The message begins its journey under the sea, and the postal network takes over above water.
That distinction also explains why the box needs special cards. Ordinary paper is a poor match for salt water, pressure and handling by a diver. The record authority specifies waterproof plastic postcards, while Susami’s guidebook directs visitors to use a dedicated postcard. The documented practice is specific: divers use the approved waterproof card for this unusual collection point.
The box’s depth is impressive, but its design is practical. It is anchored in a shallow coastal dive rather than suspended in the open ocean. A diver can reach it, place the card through the slot and return to the surface; the collector’s work then connects the novelty to a regular post office.
Folklore, record language and what the sources actually say
Popular lore: travel stories often call Susami’s box an “underwater post office.” That phrase captures the fun of the experience, while the documented object is a postbox and collection point officially linked to the Susami post office. A diver really can send a card from the seafloor.
Record claim: Guinness World Records lists the Susami installation as the deepest underwater postbox. Record titles can change, so this article uses the scope stated by Guinness rather than treating “deepest” as a permanent fact about every underwater mailbox ever built.
Speculation: the sources establish a functioning local mail point, special waterproof cards, authorized collection and a 10-meter dive. They leave the mailbox’s original motivation and each card’s exact route open, which keeps the documented story precise.
Why build a mailbox underwater?
Because a place can make an ordinary act memorable. Susami’s guidebook presents the mailbox with its diving programs and seaside activities, showing how the installation fits into the town’s public identity. A postcard is already a small invitation to imagine another place; posting one from below the surface turns the trip itself into part of the message.
The result is a peculiar meeting of systems: a metal box designed for salt water, a diver working in a calm bay, a waterproof card and a conventional post office on land. The route is only a few minutes long, but it makes the ocean feel like an address.
Sources
- Guinness World Records: Deepest postbox underwater — record scope, depth, waterproof cards and authorized collection.
- Susami Town tourism guidebook (2024) — local description of the 10-meter underwater post and dedicated postcard experience.
- Susami Town official website — municipal context and official locality information.