Your grass looks very repetitive.
And ropes are usually either straight or curved only once. Else it looks like they are really old and about to break.
Your dirt has no outline or shading, making it more difficult to add a background. I see you are trying to keep the color count low, but you could use the brown from the flags, the one from the character hair and boots and the green-brown from his belt. The same green-brown could probalby be used in the grass, and his blue eye could be used in the waterfall. Don't waste colors for a single pixel in the whole picture !
Also the ropes colors could be used as an highlight on the dirt, too.

And what are these yellow things in the dirt ? diamonds cutted in a piexe of gold ? If so, they should shine a bit more. You could use a pattern similar to the one in the red things (are they hearts, or apples ?)

Your dirt tiles need a top and a bottom. Use your shading for that.

There is no feeling of undergroundness at all here, in my op.
Try to use some games set in mines, sewers or caves as inspiration.
No grass grows undgr, there are specific light issues, there are many typical elements you can use (stalactites, damp walls, bats, arches, shafts, rungs, pipes, crystals, rats, you name it).
What kind of undgr place is it supposed to be?
If you would be more specific, I could link you to some ref material, in a week or so.