Firstly i'm no programmer, and i don't know how much easier this method would be to implement. This is just my thoughts on tunneling.
I've read a few posts about creating tunnels or mines, and the consensus seems to be it would take a large amount of work to implement. From what i understand, terraforming land involves raising or lowering the playing field. The current game mechanics don't allow for horizontal distortions.
So what if we were just able to designate an area to warp? What i propose is that if we want to dig into a mountain side or cliff we designate a specific area, in a similar fashion to how buildings are constructed. Within that area you can dig horizontal, into the mountain. Think of it like building a door, except your putting that door onto the mountain and you dig into that door. From there, players can expand the space inside the "tunnel" in all directions. A simple idea on how to "tunnel" would be to dig in the direction the avatar is facing. To dig down, the terraforming skill mechanics could be used.
For realism, every time a character "digs" and widens the space inside the tunnel. soil, rocks, minerals etc are added to the materials carried on hand (similar to how logs are carried around on your back). The model that shows on the players back could be a sack (that may need to be equipped). "Earth" that was excavated would have to be put somewhere, so it gives the player more challenge with dealing with endurance. If you just dump the "earth" inside your cave you create mounds similar to terraforming a hill. Precious materials like gems can be deposited into players bags automatically. Also for added challenge, support beams must be placed periodically. Otherwise you face being caved in. Im not sure how light will be dealt with in the tunnels, but the idea is that the farther you are from the entrance the less illuminated the area you are is. So creating torches and other light sources are needed.
Unlike terraforming that applies to almost all topside areas, this "tunneling" skill only applies to a small area that the player designates. All the rules of "tunneling" would apply only to the areas inside the designated "dig site".
The problem i see is how to implement anomalies. For example, what if we want to run into a cavern or an ore vein? How could that happen if everything inside the tunnel, is just space made by expanding the area inside the "door" to the tunnel? Caverns could be underground "bubbles" that players can connect their tunnels too i guess. That would complicate things, since those caverns would have to be designed and placed just like all the hills, lakes, rivers topside.
I played with the terraforming tools the other night. I was sad I couldn't dig very deep and sad (but not surprised) that I couldn't create my own small cave. I really like the idea of being able to dig tunnels, and if mining is introduced, so much the better, but either it should take a ridiculously long time and a lot of effort and a lot of tool wear and tear to tunnel into solid rock, or it should be restricted to homesteads and tribeland, or even both. Otherwise, you won't be able to find a mountain view that doesn't look like swiss cheese.
Your right ifireallymust, if abused the landscape could look like the moon. Tools may determine where exactly you can set up a tunnel entrance. Tribal/Homestead tunnel entrance restrictions are a good check against over excavation too. My op was really just to get the idea out there, and was more of the method to implement the tunneling system.