Descend the stone steps and the city above vanishes — replaced by an eerie underground forest of 336 marble columns reflected in still, dark water. Built by Justinian in 532 to store the city's water supply, the cistern lay forgotten for centuries until a scholar noticed locals lowering buckets through their basement floors. Two Medusa heads serve as column bases in the far corner — one sideways, one upside down — and nobody quite knows why.