I propose that we add systems of procedurally generated underground caverns, as well as new machinery and new enemies that are better suited to them.
- Seismic Surveyor
Used to find where caves are by pounding the ground. Similar to Radar.
- Shaft Boring Machine
Used to first give access to tunnels.
Used to allow transport belts, rails, and players easy access to tunnel system.
- Tunnel Boring Machine
Used to create new tunnels in a straight line. Can also maybe place transport belts behind to handle byproducts.
Smaller, more maneuverable versions of trains better suited to narrow tunnels.
- Tunnel walls can be mined slowly by hand. Resources are found in the walls, and also on the ground underneath them once mined.
- Resources are much richer underground, to encourage players to explore and make use of tunnels.
- Certain caves are actually alien burrows, with rooms and spawners.