The Jungle Beneath
Of course the name varies by region, and some areas may not have even heard of it. In most areas it is so far below the surface as to be completely inaccessible, with many many levels of twisting caves in between, but in some areas it comes very close to the surface.
But what is it like? How can there be a jungle underground? To put it simply, close proximity to lava flows keeps the ground warm, damp and nutritious enough to allow for abundant plant life. Of course since there is no sun, there is no photosynthesis, so plants vary widely in color(though white is by far the most common), with bioluminescence a common source of strange and odd colored light.
No jungle is complete without horrific beasties ready to tear the unwary limb from limb, right? Now this is still just a thought, I haven’t decided for sure yet, but how about dinosaurs? mostly smaller versions, albino, and some with bioluminescence. Imagine a t-rex with an angler fish’s decoration. Or a swarm of raptors with pale white bodies and glowing frills. Some of them might even use the light as a signal, and are able to change it based on their emotions.
Another creature of the underjungle, is a species of ape, one of the reasons so few explorers live long in the area. Basically their vision is so sensitive to uv, that even a small fire causes them pain. And they don’t really have a “fight or flight” mode, they pretty much only have “fight”. Since they breed like rabbits, a few dozen going down in one fight is no major concern to them as a species. And most everything knows better than to mess with them, unless its big enough to kill em clean, or poisonous enough to keep their mouths shut.