One of Istanbul’s most intriguing attractions, the Basilica Cistern, or Yerebatan Sarnıcı as it’s known in Turkish, was built by Emperor Justinian I in the 6th century to store up to 80,000m3 of water and channel it to nearby palaces. Featuring hundreds of ancient Ionic and Corinthian columns, the cistern is a relic of the Byzantine Empire’s water system that visitors can explore first-hand. The cool and dark underground complex is also a great way to escape the summer heat. The cistern features two column bases of unknown origin emblazoned with the upside-down head of Medusa, leaving bewildered visitors to speculate why they were situated that way.