I made a simple image in Photoshop (1) and displaced a subdivided box with it (2). It didn't turned out as planed, but by Subdividing with Smooth and fractals (3) as distortion it gave me new ideas.
I then extruded some random clusters of polys inwards and aligned them to look like man made caves.
I then extruded some random clusters of polys inwards and aligned them to look like man made caves.