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Yerebatan Cistern or Basilica Cistern was the largest of several hundred ancient cisterns that lie beneath the city of Constantinople in Turkey.
The cistern, located 150 meters southwest of the
Hagia Sophia, was built in the 6th century, during the reign of Byzantine Emperor Justinian I.
Niccolò Polo had the first of the Assassin Altaïr Ibn-La'Ahad's Masyaf Keys hidden in the Yerebatan Cistern in 1257. Later, in 1511, the Italian Mentor Ezio Auditore entered the cistern via a secret passage in the old Polo trading post, then a bookshop run by Sofia Sartor.
There, Ezio found that the Byzantine
Templars had been searching for the Key for thirteen months, without success. Stealthily making his way through the Yerebatan Cistern, Ezio recovered the Key, as well as a map to the location of the other keys.
Yerebatan is a Turkish word meaning "sunken", and the place is also known as Yerebatan Sarayı ("Sunken Palace") and Yerebatan Sarnıcı ("Sunken Cistern").
Artwork of the cistern
Ezio gazing at the "Peacock-eyed" column