Tunnelling machines
IIRC, the huge machines that actually dug the tunnels were buried in side passages, since it would have been too difficult to remove them or back them up all the way to their respective starting points.
In a world where manufacturing such machines will not be possible for perhaps decades, I can see some sort of mission to try to recover them sometime after the Twilight world stabilizes. Of course this might involve a submersible vehicle and lots of pumping equipment.
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"Let's roll." Todd Beamer, aboard United Flight 93 over western Pennsylvania, September 11, 2001.
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