Not in the slightest sarcastic. A realistic proposal, even if not done by the US government, but by another nation or even a private business interest.
Why did you think it was not genuine? It is pretty much existing technology, and would save a huge amount of money in the long run.
Oh little things like the mass of a tunneling machine, the mass of a nuclear power plant, the fact that no reactors exist that are capable of running without human supervision, the lack of lifting capability into even near earth orbit for the anticipated mass, then the lack of any propulsion system that will get more than a few hundred kilos to Mars, the iffy nature of Mars landings. The public climate against lofting nuclear reactor into space, the total lack of anything worth obtaining when you get there, the inability to return even a few KG sample from the martian surface, the fact that the only reason to put a tunneling machine on mars would be to make tunnels and the only reason for tunnels would be to have people in them, the fact that there doesn't exist the capability to put people on Mars alive let alone return them, the staggering cost of such a mission without the lisghtest economic return. Little things like that led me to believe when you used the word "sensible" you were being sarcastic.
It is pretty much existing technology
Yes I just saw a special on nuclear powered tunneling machines at Walmart on black frids (Now that was sarcasm)
would save a huge amount of money in the long run
Saving even more money would be not doing it al all.