The Subterranean Water Fortress beneath the Oppian Hill

The Sette Sale, a water fortress on the Oppian Hill in Rome, consists of nine massive compartments. Constructed over 2000 years ago, the reservoir was essential to the Roman Empire's water supply system, feeding the Baths of Trajan with over 8 million liters of water. Water was fed to the reservoir via a branch aqueduct on the Esquiline Hill. The reservoir's orientation resembles the nearby Baths, but brick stamps prove it was built concurrently with the Baths. The reservoir has two levels: the lower level is a cryptoporticus supporting the reservoir above, while the upper level channels water pressurized enough to supply the Baths. The walls of the compartments were lined with waterproof plaster, and the cistern was partially underground, with the four walls anchored by an embankment.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons