Hidden beneath the streets a short walk from Hagia Sophia lies the largest of the Byzantine cisterns that once stored water for the imperial city, built under Justinian in the sixth century. Descending the stairway into the cool, cathedral-like dark is a genuine surprise in the middle of a hot day: 336 marble columns, many salvaged from older Roman buildings, march away in rows over a shallow sheet of water patrolled by carp. At the far corner sit the two celebrated Medusa heads used as column bases, one upside down and one on its side, their origin still a mystery. A recent restoration added dramatic lighting, sculpture and raised walkways, so it now feels part monument, part art installation. It is compact, atmospheric and blissfully cool. Come first thing to avoid the queues that build by late morning, and mind the damp, uneven footing underfoot.
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