I have a large cave area near my spawn point. I noticed that it was like a shadow area that I sometimes see in an extreme hills area until light sources are put there, except this was in a desert and it was going down into the ground. Going down I can see that this exists in only a specific area. I am able to see in cavern offshoots and nearby areas, however even if I try to place torches in this area they will not light this area and I lose the torch.