Well I have not breached it either but in my last world I kept seeing "Giant cave crab is caught in flames x20" (or something like that). So I used DFreveal and went looking for what it was, and basically it was some cave crabs that had spawned in an underground structure which were attacking clowns. On another world I used reveal and unpaused, and as soon as clowns spawned they started attacking random cave crabs in HFS. It's quite odd...
EDIT: looked in the raws and noticed this
I'm not sure what impact this would have or if it would do anything, but doesn't this mean they will spawn in a cavern from level 1 to level 5 and since there is only 3 cavern's does HFS count as the fourth?