I have questions

Is this a stand-alone piece, a mockup, a game background? Is there going to be something in the foreground, sprites or other?
Why the long narrow format? Is it part of a larger image?
I'm asking because
unless there's a reason, the composition is not so good.
This will likely not read as a cave, because all we see in this narrow frame is a patch of ground and the base of a wall; there is no idea of an inner space surrounding these elements, no depth, darkness, mysterious passageways.
The frame is cut in two equal halves, and is mostly empty except for the piles of bones at both ends: the viewer's eye is lost, unguided and not knowing what to do with the empty space.
Again, unless reasons, I think you need to redefine all these major options before going on. Check references (lots!). Visit a cave if you can!
There will also arise the question of light. Caves are dark except near the entrance (and in that case the light is very specific); you'll have to define a light source that will likely appear in the image: explorer's torch, phosphorescent fungus, vampire eyes or whatever

It won't read as underground if you shade it as you would a wall and floor in a building or outside, with low contrast, soft light and no shadows.