Mt Ōyama in the south of Okinoerabu island shelters a vast system of underground limestone caves. On the southwest slope, a 1km section is open to the public, and lit up to show off its impressive calcite decorations: stalactites, stalagmites, cave pearls. On the southeast slope, it is possible to have an unforgettable caving experience (accompanied by a guide), as you move through the galleries, blue-tinted gours and natural cathedrals.