The Wall could also close down these tunnels.
Just run a network of seismic sensors along it. Nothing sophisticated, but reliable.
As soon as the digging starts, a location alert to CBP.
Then they can allow the enormous expenditure of resources on an asset they can terminate very late in the game. After the millions are spent.
I think it was seismic sensors that found the tunnel in the first place, since there was no US exit side.