As Reiger's team approached the fringe of the 24th layer, the tree bark-like ceiling and walls began to lose their original form.
The northern border of the 24th layer differed from the maze's central area. The ceiling, walls, and ground all gradually took on a rocky, uneven structure, changing color from brown and green to pale red, making the surroundings resemble a cave rather than a forest.
This was due to the proximity of the food supply cache.
While there may be slight variations, the underground city's food supply caches are fundamentally cave structures. Deep inside, there is a massive quartz-like rock pillar, from which a highly nutritious liquid seeps. The monsters sustain themselves by drinking this, replenishing their energy and nutrients.