Bones

- Some places near the surface have creatures living in them that are found on the surface itself, too.  Large creatures similar to bears and packs of animals similar to wolves make their lairs there. Sometimes only hundreds of feet from an entrance, yet often in the darkest depths.  Down to where the light starts to fade, or even fades absolutely to enveloping darkness.  After that the only denizens regularly encountered are those that dwell there most if not all of their lives.
- But this is more than a vast network of caves, tunnels, caverns and chasms.  Yes – enormous open landscapes that stretch up and out further than can be seen into not all black inkyness.  Because while many of the ways – especially near the surface – are dark, there are countless areas illuminated by flora and fauna that have adapted and evolved the ability to give off varying shades, hues and colors of light of likewise various strengths of luminosity.  One may encounter a huge forest of glowing mushrooms hundreds of feet high amidst a quiet lake which is only a foot deep at most, though spanning thousands of square yards.  Creatures resembling and behaving like lions and boars vie for control of another area, but they have bright, glowing pink or indigo or turquoise stripes or dots (or both!) all over their bodies.  And yet another area is simply a lake so wide and deep that none know its true size – most likely due to the rarely satiated appetite of supposed leviathans within.