What you are entering is a system of five cisterns for collecting rainwater. They are typical bell-shaped cisterns carved into the limestone rock, starting from a small opening and gradually widening towards the bottom, assuming their characteristic shape. The walls are covered with “cocciopesto,” a mixture of clay fragments (from tiles and amphorae) mixed with lime and beaten, which served to make the interior of the cistern waterproof. As you can easily see, the cisterns are located at different heights and have different volumes. When the water level exceeded the maximum capacity, it was collected from nearby cisterns through overflow channels. These channels are no longer visible in the hypogeal cisterns of San Giorgio, as they have been used as guides to create connecting corridors between the different cisterns, which are now accessible for visitation.
Since the concave bottom of the cisterns, which gives them their typical bell shape, allowed sediment normally present in rainwater to settle, the water that reached the next cistern was cleaner. The average volume of water collected in each cistern was about 36 cubic meters, equivalent to approximately 36,000 liters of water.
This system of interconnected cisterns, aimed at collecting larger volumes of water, is not an isolated occurrence. On the contrary, there is a large number of cisterns within the Sassi districts. This widespread presence demonstrates that the productive use of water predates its use for habitation. The cisterns, in fact, outnumber the inhabited caves and the need for drinking water, indicating an organization of spaces for agricultural gardens with terraces carved into the stone. Later, the expansion of habitation reduced space for agriculture, and many of the same cistern cavities underwent transformations to be repurposed for other uses, such as the last cistern in the water collection system of the San Giorgio complex, which was later transformed into a sort of pantry for food storage.