Samcheok, Gangwon Province
Hwanseongul, one of the largest limestone caves in Asia, near Samcheok, created 530,000 years ago, with almost 2 kilometres of steel stairways. The cave's total length is 6.2 kilometres but only 1.6 kilometres is open to the public.
Photo by Ludo Kuipers, Sat May 10, 2014