Keya no Oto is a cave made from rare rock formations created via sea erosion which is located in Itoshima area of Fukuoka. A beautiful site to behold as you approach via boat, the huge basalt rock formation is 210ft (64 meters) high, 287ft (90 meters) long, and 33ft (10 meters) wide and is one of the biggest of its kind in Japan. The ferry ride to the cave takes around 30 minutes and is definitely worth the visit!