Sandanbeki Cave (三段壁洞窟) in Shirahama, Wakayama Prefecture, will host a summer-limited mystery hunt event from July 18 (Saturday) to August 31 (Monday), 2026. The event is titled "謎解き宝探し 熊野水軍の残した秘宝を探せ！！" (Mystery Treasure Hunt: Find the Treasure Left by the 熊野水軍!) and turns the cave into an immersive educational adventure for families and friends.
The concept draws on the legend that the 熊野水軍 (Kumano Suigun) — the naval force celebrated in the Genpei War — once hid their ships inside the cliffs at Sandanbeki. Participants explore the cave, collect clues, and solve puzzles to uncover the location of the navy's fabled hidden treasure. Walking through the actual site adds a layer of authenticity: the puzzles are woven into the cave's natural features, history, and folklore, turning the visit into what the organizers call an "edutainment tourism experience."
Three Difficulty Levels Debut This Year
A major addition for 2026 is a choice of three difficulty levels, making the event accessible to everyone from young first-timers to seasoned puzzle enthusiasts:
- かけだし探検隊 (Beginner Explorer) — An entry-level course suitable for children and mystery-solving beginners.
- ベテラン探検隊 (Veteran Explorer) — A more challenging course for participants who enjoy putting their thinking caps on.
- 伝説の探検隊 (Legendary Explorer) — The hardest tier, designed to impress even dedicated mystery fans.
The "Veteran Explorer" and "Legendary Explorer" puzzles were created by Hard Nuts (ハードナッツ), a puzzle-design group formed by Kyoto University students. Originally founded in 2015 by members of the medical faculty with their debut production "Escape from the Human Body," Hard Nuts has since staged original mystery events across the Kansai region and beyond. The group's name reflects their philosophy: a hard nut is difficult to crack, but once it does, what's inside is delicious and rewarding.
How the Event Works
At the cave entrance, each participant receives a "treasure map" (the puzzle sheet for their chosen difficulty level). They then make their way through the approximately 200-meter cave passage, observing the surroundings, gathering clues, and solving the mystery step by step. The route is designed so that natural curiosity about the cave's geology, history, and legends drives the discovery process.
Participants who complete the mystery successfully become eligible for a prize lottery, with rewards including popular gaming consoles, local Shirahama specialty products, and original event merchandise.
CAVE ACADEMY Project
The mystery event is one part of Sandanbeki Cave's broader "CAVE ACADEMY" initiative, which aims to reposition the cave as a hands-on learning destination that fuses tourism, education, and regional culture. The project is building partnerships with educational institutions and local organizations to create experiences that go beyond conventional sightseeing.
Event Details
- Dates: July 18 (Saturday) to August 31 (Monday), 2026
- Hours: 8:00 AM to 5:00 PM (last entry 4:50 PM)
- Venue: Inside Sandanbeki Cave (a cave entry ticket is required)
- Admission: Adults (middle school students and above) ¥1,500 / Children (elementary school students and above) ¥750 / Infants free
About Sandanbeki Cave
Sandanbeki Cave is a legendary site at one of the Shirahama area's most iconic sea cliffs. Dating back to the Heian period, it is traditionally said to have been used by the 熊野水軍 as a secret harbor for their vessels. Inside the cave, one of Japan's largest bronze statues of 牟婁大辯才天 (Muro Daibenzaiten) stands watch, and the roughly 200-meter passage features reconstructions of a guardhouse and other structures based on historical records. Visitors descend to the cave via a dedicated cliff-side elevator and can observe the dramatic sight and sound of waves surging into the cavern.
