Basilica Cistern was built to survive a siege — an underground lake that could keep Constantinople in water when its aqueducts were cut. Emperor Justinian ordered it dug in the sixth century, beneath a public square where a great basilica once stood, which is how it took its name. Thousands of laborers raised 336 marble columns in a strict grid, most of them scavenged from older Roman ruins, so no two quite match — Corinthian capitals stand beside plain shafts. Fed by water carried from forests nearly twenty kilometres north, the chamber could hold tens of thousands of tonnes.
You descend a stone stair and the noise of the street falls away. The air is cool, the light is dim, and water drips somewhere the whole time you are down there. Raised walkways carry you over shallow water where pale fish drift between the columns, each one doubled in the reflection. In a far corner sit two carved Medusa heads, one on her side and one upside down, reused as column bases for reasons no one fully knows. The cistern has since been restored and relit, and it serves today both as a historic monument and as a venue for art installations, with a shallow layer of water still covering the floor.