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.

There are flowstones and a subterranean lake.

A Trip to Iwate Archived 2018-01-17 at the Wayback Machine Japan Travel Guide Morioka-Hachimantai Tourist Advisor

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).