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With no light but that from a burning torch, French physician Petrus Gyllius discovered why buckets of fresh water drawn from strange wells near the palaces of Istanbul would sometime return swimming with fish.

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History books explain that, in 1545, Gyllius found a set of stairs disappearing beneath some ramshackle wooden buildings behind the city’s gigantic mosque, the Hagia Sophia.

Following them down to 30 feet beneath the streets of Istanbul, he found a sunken palace and floated through its forest of 336 towering columns on a row boat.

The elaborate reservoir he discovered, today known as the Basilica Cistern, was then already about 1,000 years old. But more surprising still, was that it was portal to even older epochs of Istanbul, once known as Byzantium and, later, Constantinople.

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Beneath a cathedral-like ceiling, the ruins of pre-Christian pagan temples had been reused as building blocks for the mishmash of columns — including two stone Medusa heads propping up tall pillars — layers of history that had disappeared below the still surface of the artificial lake.