Nuclear Subterrene TBM
A Subterrene is a type of tunneling machine, similar to that of a Tunnel Boring Machine (TBM). A subterrene works by utilizing forward pressure and massive amounts of heat to push through rock. The front of the machine is equipped with a stationary drill tip which is kept between to 1300-1700 degrees Fahrenheit.
The molten rock leftover from drilling is immediately turned into a glass-like material and coats the inner diameter of the tunnel, creating an initial tunnel liner. Massive amounts of energy are required to heat the drill head, which is supplied via nuclear power or electricity.
All of this is in stark contrast to traditional TBMs, which grind away at rock instead, however the results are similar. Patents issued in the 1970s indicate that US scientists had planned on utilizing nuclear power to liquefy lithium metal and circulate it to the front of the machine (drill).