- Elevator which can put the cat one level lower into the ground. Upgradeable to reach 1 more level deeper with each upgrade.
- Under the ground, after digging dirt free pipes and places, same buildings can be used like at the surface. Perhaps little different color each level? And cheaper?
- No underground storm.
- find an dig resources like coal and others
- build rails and fast trains
- greenhouses with flowers and other nice plants. Build big expandable parks. No need to ware a space suit.