Paris, City of Light, really is a tale of two cities. One of them is above ground, with its beloved Eiffel Tower and Notre Dame. That's the city the world sees. And then there's the city very few us will ever see -- an underground Paris, the 'souterrain.' NPR's Jacki Lyden and National Geographic photographer Stephen Alvarez teamed up to see what lies below. (Photographs by Stephen Alvarez/National Geographic)

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