i have been working inside my World Trade Center I built, every 10-15 i hear a ghostly sound, and unsubtle shrieks any explanation? I'm getting very creeped out.
It also happens when you have a really dark area. It doesn't have to be a cave. I built a giant church and ambience usually plays out of each of the 4 bell towers.
I believe that when you build a large stone structure the game registers it as a cave, dungeon, or stronghold. This conclusion came to me when I was building my castle and heard those noises and noticed I never heard them in my wooden houses. however don't quote me on this because I didn't research it or anything its just a hypothesis.
Or when your above a cave system or abandoned mineshaft. More common over ravines in my game.