This commercial cave is located in the center of the town of Samcheok. The provide helmets, but if you bring your own, they let you use them!
Walking down the entrance at Cheongok Cave in Samcheok.
They had a pretty cool 3D map of the cave!
You can visit the Korean Tourism website for this cave here:
http://english.visitkorea.or.kr/enu/ATR/SI_EN_3_1_1_1.jsp?cid=789931