The underground forest (地下森林) lies about twelve kilometres from the northern gate of Changbaishan and is one of the reserve's less-visited and more unusual sights. A collapsed lava tube created a depression several metres below the surrounding landscape, and the forest that has grown within it over millennia occupies a space that feels genuinely subterranean — you look down into a dense green canopy rather than up at it.
The site has a boardwalk path around the rim and descending into the depression, allowing a quiet hour of walking among the old-growth trees at the bottom. The contrast between the volcanic landscape at the crater summit and the dense, moss-covered forest floor here is substantial. Fewer visitors make the trip out here compared to the main summit attractions, which is itself a reason to come — the atmosphere is quiet enough to hear the forest rather than the crowd.