On September 11, 2001, Taliban and al-Qaeda troops hunkered down in a labyrinth of caverns and tunnels inside the mountains of Afghanistan. Half a world away, as the first hijacked airliner smashed into the World Trade Center, US Air Force fighter planes scrambled on orders issued from a command center deep within Cheyenne Mountain in Colorado. Have we really come very far since our prehistoric ancestors camped in caves to escape animals and enemies? A look at the history of underground defense installations says we have.