My favorite thing in Istanbul was the Roman Cistern. I found it by accident when I tried to avoid an annoying street salesman by following a group of tourist into a non-descript building. Basically the cistern is a 9800 square meter room below the city that the Romans used to store water they brought down from the mountains. The air in the cistern was a cool 60°F compared to outside which was over 95°F and water dripped down on us from the ceiling. Though built in 532 AD, it was eventually forgotten until 1545 when an archeologist heard stories about people pulling buckets of fresh water from holes in their basement. Sometimes they even pulled up fish. The cistern was renovated in 1985 and opened to the public in 1987. Inside are boardwalks that allow you to walk above about a foot of water that covers the floor. The arches, domed ceilings, soft music, and darkened atmosphere help make this an interesting visit. In the far corner are some mysterious columns that sit above granite carvings of Medusa which were imported from Rome when the cistern was built.