I was digging my another base underground and I thought that it would be great if on planets we would have cave systems. It will make things easier and harder :)
You want this part to be digged out (for stone) sorry there is already empty space there.
Maybe something simmilar that there is on the Asteroids.
Could be that the caves are more frequent under mountains?
This means that we could have more usefullness for rovers. In this I would also not limit the depth of the resources on planet.