The Gosu Cave (고수동굴 - Kosu is also still found as an alternate spelling) is a huge limestone cave near Danyang (North Chungcheong Province), South Korea formed over 450 million years ago. It is among the best known natural caves in Korea, called the "underground palace" because of its breathtaking natural beauty. Gosu Cave is listed as the country's "Natural Monument No. 256".