Basilica Cistern, Istanbul
The cistern was built by Emperor Justinian in 532 to store water for the Great Palace and surrounding buildings. From Lonely Planet: "The cistern is 65m wide and 143m long, and its roof is supported by 336 columns arranged in 12 rows. It once held 80,000 cu metres of water, pumped and delivered through nearly 20km of aqueducts."
Operatic music plays over the loudspeakers in the cistern, and there are big black gold carps swimming in the shallow water below.