The Basilica Cistern is a must-see on any Istanbul itinerary. If you take a tour of the city, you may see major mosques, houses, and churches, but this is a truly unique experience. It is located right across the street from the entrance to Aya Sofya and the Blue Mosque.
It is really cool to walk underground and see where water was stored in olden days.
Insider's tip: Visit the cistern in the late afternoon when Istanbul is hottest. The temperature in the cistern is usually up to 20°F cooler than at ground level. And don't miss the Medusa heads!