Good point. An underground cavern might be fatal to the entire bore plan. I talked to a well driller once who said he lost an expensive bit when he drilled down into a cavern, or void of some sort and lost the bit.
Here’s an article with photos of a large boulder (34’ x 75’) that is on the ground surface about 50 miles north of the tunnel.
A large cavern is very unlikely in the geology where the boring is. And it would not “obstruct” the boring - but they obviously wouldn’t be able to construct the tunnel.
Although it would be REALLY COOL to have to drive into the tunnel, go a mile, then drive across a bridge across the cavern, and then into the tunnel again on the other side! Have a “view point” on the edge of the cavern where you could get out and stretch your legs, and maybe have walkways to go exploring. Sort of a Jules Vern thing!