Pont de l'Alma, rive gauche, face au 93 quai d'Orsay, 75007 Paris
Price: from € 2.30 per person.

If visiting the catacombs was not enough for you, you still have to visit the Parisian sewers. Maybe you'll come across the famous alligator that is supposed to live there but this seems unlikely. However you can experience a small part of this vast network and learn more about its construction and operation.