Beneath the busy neighborhood of Sultanahmet in Istanbul lies the Yerebatan Sarnici, or Basilica Cistern, an astonishing feat of 6th-century Byzantine hydraulic engineering. Constructed during the reign of Emperor Justinian I after the Nika Riots of 532 AD, this subterranean chamber was built to supply filtered water to the Great Palace of Constantinople. Measuring roughly 138 meters long and 65 meters wide, the cavernous space can hold up to 80,000 cubic meters of water.
The vault is supported by a majestic grid of 336 marble columns arranged in 12 rows of 28, earning it the moniker of "Sunken Palace." Because the cistern was built rapidly under military and civic pressure, Byzantine builders repurposed marble columns, pedestals, and capitals scavenged from ruined pagan temples across the Roman Empire. This practice, known as spolia, created an eclectic subterranean gallery featuring Corinthian, Ionic, and Doric styles alongside uniquely carved "peacock eye" columns.
The most famous architectural mystery lies in the northwestern corner, where two massive column bases are sculpted into the likeness of Medusa, the Gorgon of Greek mythology. One Medusa head is oriented sideways, while the other is positioned completely upside down. Historical legend suggests that Byzantine builders inverted the heads to neutralize the mythological creature's petrifying gaze.
However, architectural historians favor a more pragmatic explanation: the carved blocks were reused purely as heavy load-bearing foundations, and turning them sideways or upside down matched the specific height requirements of the vaulted ceiling while minimizing pagan artistic reverence in a Christian empire. Sealed with thick brick walls coated in waterproof mortar, the cistern remains a mesmerizing triumph of ancient resourcefulness.