1. Basilica Cistern
The largest and probably the oldest cistern that stands beneath the city of Istanbul. It was built in the 6th century while the city was still called Constantinople and it was under the control of the Byzantine Empire. The cistern is full with Medusa statues which make the places even more interesting. The cistern is also really close to the most famous tourist destination in Istanbul – Hagia Sophia. Interesting place to visit that is fairly unknown to the international tourists.