Despite its name, the Underground Forest isn’t below the earth; instead it has grown within volcano craters that erupted some 10,000 years ago, giving the appearance of trees sinking into the earth. Hiking around the thick pine forest and several of the 10 craters takes about an hour.
The forest is 50km from Jingpo Lake. Some day tours include it in their itinerary. Otherwise, you have to take a bus from the north gate of Jingpo (¥40 return, one hour), which is doable but very tight if you only have a day at the lake.