By Kurmanbek - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=122644443
It's the year 1545. A French scholar named Petrus is strolling down the streets of Constantinople. He notices a guy selling fish in front of his house.
"Wow! This fish is so fresh," Petrus says.
"Yeah. I just caught it in my basement."
Petrus gives him a dazed look. "Huh?"
The guy goes on to explain that people living around here have discovered if they dig a little well in their cellars, they have fresh water...and can even catch fish.
Fresh water in their houses?
In the 16th century?
Petrus investigates this mystery.
In the year 532, Roman Emperor Justinian had 7000 slaves convert and build a cistern atop an old basilica. Able to hold 17.5 million gallons of water, it's nickname to this day is: The Sunken Palace.
Originally, this basilica cistern provided all the water needed for the "Great Palace". If Constantinople was ever under siege, their water supply was protected.
The fish were a clever addition.
If the fish died, you'd know the water supply to the Palace had been poisoned.
When turned into a cistern, they used whatever they could find from even older ruins. An old Medusa head fits perfectly to prop up this column.
By Metuboy - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=122151837