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 and 1700 °F.
The molten rock left over 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 through nuclear power or electricity.
All of this is in contrast to traditional TBMs, which grind away at rock instead, but the results are similar. Patents issued in the 1970s show that U.S. scientists had planned on utilizing nuclear power to liquefy lithium metal and circulate it to the front of the machine.
The process is supposed to be cheaper and the benefit of the initial tunnel liner also helps reduce costs as it is beneficial as is. The machines use a lot of energy, though, and their safety and reliability is unknown.