Milmu

Gosu Cave (Korean: 고수동굴), alternatively Kosu Cave, 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, and is known as the "underground palace". Gosu Cave is listed as the country's "Natural Monument No. 256".

"Gosu" means "field of tall reeds," and comes from the fact that the area used to be filled with fields of tall grasses. After the cave was discovered in the early 1970s, rough stone instruments were excavated from the cave's entrance, showing that this used to be a home to prehistoric people.