We discussed a nuclear tunnel boring machine a few years back either here on on another list. I did a search for this on Google and there is alot of crap, but I did find this excellent science paper on "Micro-Thermonuclear Plasma Tunneling
by Rock Melting" some excellent information and lots of details
https://pdfs.semanticscholar.org/1d8...0cf06af911.pdf
I do believe melting rock is the best thing to do.