As you and your friends squeeze through, you wish you would have brought a flashlight. You've found the abandoned entrance to a very old tunnel. You've heard of the famous limestone mines - and now you've discovered a secret opening. What lies below? What's behind the myths and legends? Will you come back?
National Geographic had a compelling article on the mines below Paris. You should read it. There was an paragraph in the article that caught my attention. It mentioned that there were access doors to these mines located in schools! Imagine that - wandering through your school and discovering a door to the limestone mines. It's the making of a great adventure story. If I were a writer, I'd play around with that concept.