Akkadō (安家洞), 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).

[1] 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.

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