Here is another background fantasy sound atmosphere that I wanted to share with you.
This is the inside of a deep artificial tunnel, most likely a sewers. We can hear a lot of water dripping from several sources. A few gusts of the howling wind from time to time. Hatches opening and pouring out a thick and slimy content.
Mice and bats seem to be the main tenants of these corridors.
There is a constant rumble in the background that seems to be coming from deep depths of the earth, giving the impression of endless tunnels. The atmosphere is thick, wet and scary.