No one really has an explanation for the two massive Medusa heads that support columns in the Basilica Cistern, a cathedral-sized reservoir underneath the city of Istanbul.

The Medusa, of course, is a monster from Greek mythology: a winged woman with living snakes for hair; anyone who looked at her was turned to stone.

The structure—the largest of hundreds under the city—was built in the 6th century, during the reign of Byzantine Emperor Justinian I and provided a water filtration system for his palace and other buildings.

The simplest explanation is that the heads were repurposed remains that just happened to be the right height—and no particular meaning was indicated by their orientation. A traditional account is that inverting the heads was meant to negate the power of the medusa; similarly, they may have been placed to show early Christians’ attitude toward the mythological religion of the Greeks.