One of the cool things about Jeju is that it was once a volcano and you can find remnants of this all around the island. Manjanggulsa Cave is one of the longest lava tubes in the world. We decided that this would be something very interesting to see and so we hopped on a bus. When we arrived, we actually had to walk for a bit near some fields, and in very high heat. When we reached the cave we were delighted to learn that as you get further and further into the lava tube the air is much cooler. So in addition to seeing something amazing, we also got a nice relief from the heat.
It is very dark in the cave so it was hard to take good pictures. There are different signs everywhere that explain what everything is (in English and Korean). The cave also has lights, illuminating a path for visitors to walk on, I would advise wearing shoes (not flip flops) as there are several areas of water (not deep, but puddles) and you may have to walk through some.
At the very end of the cave lies the largest lava column in the world! This is formed when the lava flows from the ceiling down into the tube.