Akkadō

Akkadō, located in Iwaizumi, Iwate Prefecture, in the Tōhoku region of northern Japan, is the longest limestone cavern in Japan, with a measured length of 23,702 metres (77, 762 ft). The cave system was designated as a natural monument by the Japanese government in 1980. Since 1991, a portion of the caverns have been open to the public for part of the year.