What Lies Beneath: Eighty-two feet below Istanbul is a 450-foot-long, 213-foot-wide former royal reservoir. This Byzantine engineering feat has 336 mismatched columns taken from the ruins of the buildings conquered by Constantine the Great, who built the cistern in the 4th century A.D.
How Did It Get There: After Constantinople fell to the Ottomans in 1453, the cistern was all but forgotten, except by the residents whose houses sat on top. Rediscovered in 1545 by a Frenchman searching for Byzantine antiques, it briefly was reintroduced to everyday life, but fell into disrepair once more, not to be used again till 1987 when it was reopened as a tourist attraction.
Cool Bonus: Two stone Medusa heads, pilfered from an ancient pagan site and used in construction, are placed in a disrespectful fashion—one upside down and one on its right side—to mark the builders’ belief in Christianity.