Come out or stay down.
The big ones usually are taken apart. Workers unbolt the heavy steel. Cranes elevate the cutterhead to the surface.
Parts go back to the factory. Workers replace the worn teeth and seals. The machine resumes its digging again in a different city.
Earth sometimes keeps them down there.
A tunnel could end deep below, and here it costs too much to get it back.
Motors, wires, and precious metals. The crew strip all of that. Drive the machine into a short side tunnel.
That hefty steel shell, left behind and then they pour concrete over remains.
Its tomb now.
The beast merges with the wall for eternity. It die where it stops.
Cost much less to leave the iron in the ground than to bring it back. So the beast lies quiet in the stone it broke.