Yerebatan Cistern (Basilica Cistern) - Istanbul Turkey

The Royal Cistern, also known as Yerebatan Saray ("underground palace"), is the largest surviving Byzantine cistern in Istanbul. This covered underground tank had a capacity of 78,000 cubic metres of water and is located to the southwest of Hagia Sophia. It was originally under the Royal Lodge, which no longer survives. In the 6th century it was rebuilt in the form it has today. It remains one of the most important and oldest public spaces in the city.