The Unknown Zone: Underground Tunnels around the world, where, why and who!

Scientists used to refer to early humans as cavemen, however a more accurate description would have been tunnelmen.  Vast tunnel systems exist throughout the planet from recent tunnel formations due to governments around the world, the military, and of course rich people who have paid to have fancy underground dwellings just in case it all goes to pot.  But we were not the first humans to do this.  As early as 12,000 years ago humans around the world went underground and it appears to be related to an extreme climate change.  Life goes in cycles, are we about to repeat this cycle?  Hint, invest in shovels.