The Pozo de El Salvador are basements and a well located in the city of Toledo, in Castile-La Mancha, Spain.
It is both well, because it emanates water of itself and also cistern as it receives rainwater through 3 holes in the roof.
It is believed that this cistern-well was blocked in its day and remained hidden since for centuries the water was a precious commodity in Toledo.
The rest drank from the contaminated streams that flowed into the Tagus.
They tried to purify the water by filtering it through pots of clay and boiling it.