3x3 Tunnel Boring Machine
Bores through water, lava, sand, gravel without issue. Builds a canopy only if there no solid blocks (in tunnels or ravines), otherwise it only builds a cobblestone floor.
It's rare, but if falling gravel allows liquid in from above, the redstone torches might break. Plug the hole and place a torch to fix it.
Be sure to provide the TBM with ample cobblestone, at least 9 dirt, 6 redstone torches, and 3 powered rails before activating.
Change the deployers on the front to use Netherrack if boring through the nether.