Inside the tunnel. It's a little hard to tell from the film, but I'm pretty sure this is the tunnel behind the entrance in the last shot. It's still in use, but the metal doors have been replaced with much more solid barriers, and better security. The film refers to 2 miles of tunnels covered with between 20 and 200 feet of rock. The depth would depend on how far back into the rim a section of tunnel was.
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