Originally posted by Intellect Inside:

It's not a voxel game. The world isn't made of cubes that can be removed or added. The terrain uses a height map. When you dig, it lowers the height at that spot. So no tunnels.

Yep - it is simply impossible to allow tunneling without some big rework of the terrain-system (which then also includes basically everything else like how objects are placed in the world or animals move), the save-system etc....