Think of underground biter nests; they would be like ants, sending diggers out and bumping into players, then finding your base and planning an assault -- they could even carve their way to your base through earth that hasn't been moved yet. To map the terrain underground, you couldn't just use a regular radar, either. You would need something like a seismic vibration detector to pick up movement and track them to their base.
Now all I want to do is make Dwarf Fortress in Factorio.
