This is National Monument No. 98, the Manjangul lava tube. From ground level, 113 steps lead downwards into the cool dampness of an underground tunnel. You don’t get any sense of scale from these photos (the best I could do without a flash), but it’s over 7 km long, and up to 18 m wide and 23 m high. So, it’s an enormous feature.
The surfaces of the tunnel bear witness to the way in which it was formed. Lava from volcanic eruptions flowed down a hillside; the surface cooled and solidified while the lava below the surface remained hot and continued to flow, leaving a hollow tunnel. Subsequent lava flows within the tunnel melted rock from the floor, making it deeper, and melted the sides, leaving a series of ‘tide marks’ depending on the depth of the lava. In places, chunks of rock were carried along by the flow, only to be deposited and coated as the lava cooled.
Notes from a Korean island 