Paris, City of Light, really is a tale of two cities. One of them is above ground, with its beloved Eiffel Tower and Arc de Triomphe. That's the city the world sees. And then there's the city very few us will ever see — an underground Paris, the souterrain. Below the city, concentrated on the south bank of the river Seine, hidden limestone quarries dating back centuries provided the stone for Paris' great monuments like Notre Dame. The mining left behind a labyrinthine maze — at least 180 miles of abandoned tunnels, secret rooms and caverns, odd wormholes barely big enough to wriggle through, running directly below some of the best-known city streets.