Danyang Ondaldonggul Cave (단양 온달동굴)

Danyang Ondaldonggul Cave is a limestone cave in Danyang, Chungcheongbuk-do, named after the legendary Goguryeo-era hero Ondal whose connection to the local fortress traditions ties the cave to Korean folkloric memory. The cave runs roughly 760 meters in total length, with a main visitor route of about 500 meters that threads through several large chambers featuring stalactites, stalagmites, flowstone formations, and a cave stream. The cave temperature stays in the high teens Celsius year-round, which makes it an unusually strong summer cooling stop and a still-comfortable winter warming stop. Above the cave, the surrounding Sobaeksan-area landscape and the nearby reconstructed Ondal Castle are part of the wider attraction. Pairs naturally with Danyang's broader cave-and-river circuit including Gosu Cave, the Danyang Maneungseki river bend, and the Danyang riverside.