I've had enemies spawn inside of walls/rocks/structures and there is no way to get to them unless you are actually able to break whatever is obscuring your line of sight to them.

Like those big copper deposits, I've had one near a ruined stone building that a bunch of greydwarves were. There was one that spawned in the middle of the copper deposit. I could hear him, but couldn't see him. I had to break through the deposit to find him.

It happens at times. Sometimes there's nothing you can do about it.

rocks can sometimes obscure openings, and if i remember i have seen submerged tunnels that you must swim into and then come out the other side in a room. Check for extended halls in swimable areas? pay attention to the map to see if you can see part of an unexplored hallway, etc.

I guess another structure nearby could do it, if things are somehow clipping through or something. It's just kind of annoying when you're trying to clear everything, especially the mines. thanks

It's a standard soundtrack. Burial chambers I cleared aeons ago, as well as troll caves, still have these ambient sounds in them.

Originally posted by MF:

It's a standard soundtrack. Burial chambers I cleared aeons ago, as well as troll caves, still have these ambient sounds in them.

It's not just sound, which are not ambient but actually the sounds of blobs jumping up and down, it's the fact that when crouched something is aware of me.

it is a bad design call or possibly an entombed mob. Both happens here. And like said by another check your perimeter as well, any down slopes nearby, return with torches so you can see clearly?, clear/level the ground near the entrance, look for any textures that may seem to be protruding to the inside of your crypt. Look for gaps or unusual wall sections. Drop a fire so you can see.

If there is another nearby underground structure you could be hearing those mobs. I have seen it where you accidentally fly out of a dungeon and all of the dungeons are just floating there, presumably lined up with where they are in the normal world. I'm not sure if they are below the world or above the world tree.

yes, all the time, its just ambient sounds.