A Forest!
There was a forest in this Underground World!
When Leon first saw this flourishing green forest, he felt dumbfounded.
His first reaction was to use a spell to check whether it was actually real. The result assured Leon and confirmed that what he saw wasn't caused by any illusion, but was instead an actual, genuine space within this Underground World.
Leon stood at the cavern entrance and scanned the boundless and dense forest. He instinctively sensed a strange and twisted smell lingering in the air. He was standing on the dividing line. Taking a step back would bring him to a hard rock layer that was painted with darkness and a weak glow of metal; while taking a step forward would let him be greeted by greenish grass that touched his ankle, and tall trees and dense vines that tightly barricaded the passage. In addition to this, there were also rose and fruit bushes.