this won't fly, but maybe something is useful:
- Mine Holes cost the same materials on all levels
- Character must "unlock" level by finding a "way" to the next one
- Once a char unlocked the level he can build a hole anywhere
The coolest way IMO to find a way to next level would be navigating rivers and sliding waterfalls to the next level.
This could have some cool animations, maybe even some stats checking(or good luck).
Player would either succeed or break canoe and wake up somewhere along the coast.