The world's largest lava cave, Manjang-gul was created centuries ago, back when Hallasan was still an active volcano. Only the first kilometer (half mile) of the 13km (8-mile) cave is open to the public, but that's enough to give you a good glimpse of its impressive rock formations and stalactites. The inside temperature is always cool no matter how hot it is outside. Although the cave is well lit, watch your step because the humidity makes the rocks slippery.