The Susami Underwater Post Box is one of the world’s most unusual mailboxes, located beneath the sea off the coast of Susami Town in Nishimuro District, Wakayama Prefecture. Sitting at a depth of approximately 10 meters in the clear blue waters, the mailbox allows divers to actually post postcards while underwater.
The bright red, cylindrical mailbox standing on the seabed looks almost like a tiny post office hidden beneath the ocean. For many years, it has been a popular destination for divers and an iconic attraction of Susami. Its fantastical appearance has also been featured in television commercials and travel programs, attracting attention from visitors both in Japan and overseas.
The underwater mailbox was created in 1999 as part of the Nanki-Kumano Experience Expo, a large tourism event designed to showcase the natural and cultural attractions of southern Wakayama.
The idea was proposed by the postmaster of Susami Post Office and brought to life with the cooperation of local diving professionals and community members.
The mailbox was installed approximately 100 meters offshore from the Karekinada coast, at a depth of around 10 meters. An old-fashioned cylindrical red mailbox was selected for the installation, creating a striking contrast with the surrounding blue sea.
In 2002, the mailbox was recognized by Guinness World Records as the “deepest underwater postbox”. Its extraordinary location has made it a unique attraction not only in Japan but also for international travelers looking for unusual experiences.
The greatest appeal of the Susami Underwater Post Box is that visitors can actually send a postcard from underwater.
Visitors write their message on a special waterproof postcard, take it underwater while diving, and place it directly into the mailbox. Local divers collect the postcards regularly and deliver them to Susami Post Office, from where they are sent throughout Japan and overseas.
A postcard that has traveled through the ocean before reaching its destination can become a particularly meaningful souvenir for the person who receives it.
The service also accommodates international airmail, allowing visitors to enjoy the romantic experience of sending a message from beneath the sea to someone anywhere in the world.
Many people use the underwater mailbox to celebrate special occasions or preserve memories of their trip. Others choose to send a postcard to themselves, creating a unique souvenir that they can rediscover after returning home.
The underwater mailbox is accessible only to divers, but visitors who do not have a diving license may still be able to experience it by joining a beginner-friendly introductory diving program.
Local diving shops offer programs designed for first-time divers, with instructors providing guidance and support throughout the experience. This makes it possible for travelers with no previous diving experience to try the underwater mailbox under professional supervision.
Waterproof postcards can be purchased locally. Visitors write their message using a special waterproof pen before taking the postcard into the water.
The moment when you descend beneath the surface, approach the bright red mailbox, and place your message inside is an experience that is difficult to find anywhere else in the world.
For many visitors, the combination of adventure, the ocean, and the simple act of sending a letter makes this one of the most memorable experiences available in Susami.
Susami has long been known as one of the leading diving destinations in the Kansai region. Influenced by the warm Kuroshio Current, the surrounding waters can offer excellent visibility and a rich marine environment.
Divers exploring the area can encounter colorful tropical fish, coral, and a variety of other marine life. This means that the underwater mailbox is not simply a destination for posting a postcard—the ocean itself is an important part of the experience.
The unique opportunity to post a letter underwater has attracted divers from across Japan and beyond. Over time, the mailbox has grown from a temporary event attraction into one of Susami’s best-known tourist attractions.
For diving enthusiasts, it offers the rare chance to combine an underwater adventure with an everyday activity that normally takes place on land.
The Susami Underwater Post Box is more than a tourist attraction. It has also become a symbol of the local community’s relationship with the sea.
Local divers, fishing organizations, and residents have worked together to maintain the mailbox over the years. Because it is exposed to seawater, corrosion, storms, and typhoons, the mailbox requires regular maintenance and replacement.
In 2007, for example, an aging mailbox was replaced with a new one, with many volunteer divers taking part in the work.
The local community has continued to care for the mailbox, replacing older equipment when necessary and ensuring that this unusual underwater attraction can continue to be enjoyed by future generations.
The efforts of these local people are an important part of the story behind the mailbox. Without their ongoing support, this remarkable attraction could not continue to operate beneath the sea.
The Susami Underwater Post Box was originally created in connection with the 1999 Nanki-Kumano Experience Expo. The event was designed to introduce visitors from across Japan to the natural environment, traditional culture, and outdoor experiences available in southern Wakayama.
Rather than being limited to a single exhibition venue, the expo used the wider Nanki-Kumano region as its stage, offering visitors a variety of experiences throughout the area.
Among these attractions, the underwater mailbox became particularly popular. Even after the expo ended, the mailbox continued to be maintained and used, eventually becoming a permanent symbol of Susami.
Today, the underwater mailbox is recognized as one of the distinctive attractions of the Nanki-Kumano region. Its combination of unusual design, ocean adventure, and local history makes it especially appealing to travelers looking for experiences beyond conventional sightseeing.
A bright red mailbox standing quietly in the blue ocean is a sight that few travelers will ever forget.
Sending a message to a loved one, writing a postcard to commemorate a special occasion, or simply mailing a letter to your future self can turn an ordinary postcard into a treasured memory.
When visiting Susami Town, take the opportunity to explore its beautiful coastline and, if conditions and diving arrangements permit, experience the remarkable underwater post box for yourself.
The Susami Underwater Post Box brings together nature, adventure, local community, and a touch of romance. It is a truly distinctive attraction where you can send a message from the ocean and create a memory that will stay with you long after your journey through Wakayama has ended.