Digging is what Tunnel Town is all about!  Have your bunnies dig to expand their burrow according to your design.  As you level up, you’ll be able to dig into deeper and deeper layers of rock and dirt underground.  As your bunnies approach a new layer, you will see the level displayed that your burrow needs to reach in order for you to continue digging deeper.  New strata are unlocked at Levels 5, 10, 15, and 20.