It never seems to get stated directly by tunneling through stable rock has to be really easy. Like if you can drill and blast your way through you just need a small crew of workers and a dump truck.

There is a half mile long tunnel in the California desert that an addled miner made with hand tools and dynamite over 38 years. Self funded and part time.

https://en.wikipedia.org/wiki/Burro_Schmidt_Tunnel

Then digging through unstable rock with water intrusion and danger of flooding the costs have to go way way up.