Mud, most of the tunnel boring machines dig through mud. Mud does not need any more fracturing, its quite easy to dig through as is. Its really hard to dig tunn

Mud, most of the tunnel boring machines dig through mud. Mud does not need any more fracturing, its quite easy to dig through as is. Its really hard to dig tunnels in it because it likes to collapse on top of you. It's like why do you use a drill and screws to hang a picture instead of a handgun some 9mm's and a nail.