From what I have read elsewhere, there have been several times that the world was going to be destroyed and some of the people had foreknowledge of impending doom. Places like Turkey had natural tunnels and huge rooms.
Other places had caves carved in mountains like in the Red Sea.
Modern systems have been bored out using tunneling machines. There is a mag levitation train system that can run the entire globe deep underground.
There are also waterways such as the one that connects Las Vegas to Puget Sound in Washington State. They can run submarines and small ships in some of the tunnels.
Now comes an opinion of mine...
What if we are clearing the D.U.M.B.s and tunnels freeing children and ridding demons in the chance that a new cataclysm is coming, like a super nova or solar flair.
Food for thought.