technically, you don't need a drop shadow(which implies light direction) as opposed to an inner/outer glow. You can make this fairly easily by making a selection so that the "outside" of the cave "rooms" is editable. Then shrink the selection by a few pixels(assuming this is the original size and not reduced) and fill a new layer with black. So it would look like the black was covering the entire walls of the cave. Then use Gaussian blur to soften it up. This layer needs to be below the rock (solid rock areas) and above the floor texture. You might need to play with the blur amount or the size of the shrunk selection to get it looking just right.