Basilica Cistern, Istanbul

This is an often missed tourist attraction that deserves to be on the itinerary of every visitor to Istanbul. It is an ancient underground cistern that was built in the 6th century. It is really incredible to walk around in, and arriving at the Medusa at the end is quite a chilling experience. Also, a bit of trivia for you, the place was used in the James Bond film "From Russia with Love". I highly recommend that you visit this place.