Empty undergrounds could be for free play mode. This brings me back to my tunnels of madness post a long time ago. These tunnels are primarily inhabited insects and can be challenging. Their locations can consist of the tunnels that hermit crabs and spiders come out on 2.1, rusty tin cans on the beach, abandoned termite nests, Mike's old college computer from 2005, etc. When cleared out these places can provide food and sometimes royal gelly. It could be an interesting thing to implement.