[gamblor956]

Elon must be pretty out-of-date on his tunneling research since most of the tunneling machines in use these days are electrical, powered directly from the local grid. (Think about it--if you're running a combustible engine in a confined space the last thing you would do is flood the area with pure oxygen.)

The thermal limits in boring isn't related to the energy source--it's related to the bore head warming up from the friction of boring. This problem is addressed by cooling the bore head with water.