Istanbul is a city of layers, with new conquerors and emperors building upon the ruins of those they sacked. This both gives Istanbul its magic, but also means that some of the beautiful ancient places we are able to enjoy now weren’t always available to the public. It’s hard to imagine that a famous tourist attraction in a major city might ever be hidden and inaccessible, but that was nearly the fate of the Basilica Cistern in Istanbul.

And that knowledge might have stayed lost, if not for a curious tourist and his strange observation.

Frenchman Peter Gyllius was visiting Istanbul in 1545 to find ancient manuscripts for the King of France. A scientist, translator, and observant traveler, he stayed in the houses of locals. It was there that he noticed that native families were accessing water through holes in their floors, which would have been strange enough, but sometimes they were also pulling up fish.

Intrigued by this custom of floor-fishing, Gyllius investigated throughout the neighborhood and eventually realized that there was a vast cistern underneath the surface. He explored the vicinity until he uncovered the ruins of the Basilica Cistern in a basement. This is the ultimate story of a tourist discovering a hidden gem on his travels. Perhaps the citizens were aware that there was something under their houses, but the scope of the system had been long-forgotten.

Initially, the Ottomans did very little with the newly-discovered cistern, using it mostly as a convenient dumping ground for their junk, but after some time the Ottomans excavated it and began to use the cistern to provide water to Topkapi Palace and other official buildings in the area. As the empire declined, the cistern again fell into disuse. It wasn’t until 1985 that the cistern was finally properly uncovered, with trucks full of mud hauled out of its elegant halls. Two years later, it was reopened to the public, and now the Basilica Cistern is one of the most popular tourist attractions in Sultanahmet. It’s one of two cisterns that are currently open to the public, out of the hundreds hiding under the surface of the city.

This is the exciting thing about Istanbul– in a city with as many layers of history as Istanbul, there’s often something significant hiding just below the surface. The ruins of the Hippodrome track are six feet below the ground in Sultanahmet. When digging under the Bosphorus Strait to construct a new metro line, workers accidentally uncovered the ancient port of Byzantium. And if an inquisitive Frenchman hadn’t been intrigued by some locals pulling fish from their floor, the Basilica Cistern might still be inaccessible underground, just another one of Istanbul’s mysteries.