Nearby Aya Sofia is the 6th century Byzantine underground Basilica cistern, with 335 massive Corinthian columns supporting the immense chamber’s fine brick vaulting. This is one of several buried into the city’s foundations, and the first to have been excavated and renovated. Thought to have been built in the 4th century by the emperor Constantine, then enlarged two centuries later, it was supplied with water from Belgrade Forest, amd supplied it to the Great Palace and Topkapi Palace.

It fell into disuse and was then restored in 1987 with the mud and water removed, and narrow raised pathways providing easy access for visitors. It is the largest covered cistern in the city, measuring 140 by 70 metres.