Chungju Hwalok Cave (충주 활옥동굴)

Developed in 1922 during the Japanese Forced Occupation, this is the only talc mine in Korea and is the largest in the East, with a recorded length of 57km, an unofficial length of 87km, and an underground vertical x_height of 711m. A 2.5km-long stretch of the cave has been developed into a tourist site, filled with light sculptures, performance venues, and health therapy zones. LED, neon, and a variety of natural lights create a mysterious atmosphere, and since the temperature stays between 11 and 15 degrees year-round, it's even more popular in summer. There is also a kayaking experience that allows visitors to see inside the cave by kayaking in a lake created by mineral water.
Note that the temperature inside the cave averages between 11 and 15 degrees Celsius, which is warm in the winter but cold in the summer, so you'll want to bring a long-sleeved jacket.