In the year 27 B.C., Marco Agrippa, the general and close friend of Emperor Augustus, established the base of the Roman fleet, known as the Classis Misenensis, at the Portus Iulius (Port Julius) in Misenum.
It was situated at the western end of the Gulf of Pozzuoli near Naples, and from there, when he was prefect in charge of the fleet, Pliny the Elder was able to observe the beginning of the fateful eruption of Mount Vesuvius that buried Pompeii and Herculaneum. He immediately set sail to try to help and, as is known, never returned.
Agrippa also had constructed an aqueduct, the Aqua Augusta, which provided water to eight cities around the Bay of Naples, including Pompeii, Herculaneum, Stabiae, and Nola.

The aqueduct, remnants of its original structure still visible in some places, begins in present-day Serino and runs for 96 kilometers to the Portus Iulius.
The main purpose of the aqueduct was therefore strategic, as it could provide water to the fleet base even if it was besieged. For this purpose, at the end of the aqueduct, on a promontory facing Cape Misenum, a large cistern was built to store all the water necessary to withstand a long siege and to supply the fleet’s ships.
It was excavated in the volcanic rock of the cliff wall and measures 15 meters in height by 72 in length and 25 in width, resulting in a capacity of 12,600 cubic meters (12.6 million liters of water).

It was constructed as a hypostyle basilica (supported by columns), with a rectangular plan and four rows of twelve cross-shaped pillars each dividing the interior space into five long aisles and thirteen courtyards.
The total of 48 columns support a barrel vault covered by a roof of opus caementicium and paved with waterproof opus signinum. Due to its resemblance to a cathedral, it has been dubbed the “cathedral of water”.
The Tuscan poet Francesco Petrarca, who visited it in the 14th century, gave it a more evocative name: Piscina Mirabilis. Of course, he could see it with the row of twelve additional small chambers with barrel vaults that had been added at the beginning of the 2nd century AD to increase capacity.

One of these chambers still contains a mosaic in the shape of a labyrinth on the floor, and a panel with white inlays of polychrome limestone tiles.
Access to the cistern was via two staircases supported by three arches, one at the northwest corner (still accessible today) and the other at the southeast.
Inside, right in the center of the central aisle, there is a basin excavated in the ground with a depth of 1.1 meters and equipped with a drain at one end. It served as a settling basin for the clarification, cleaning, and periodic emptying of the cistern.

Water was extracted from outside through the openings in the barrel vaults, pumped by hydraulic devices placed on the roof.
The eruption of Mount Vesuvius in 79 AD, which buried Pompeii, Herculaneum, and Stabiae, did not affect the aqueduct, and water continued to fill the Piscina Mirabilis.
But four centuries later, in 472 AD, a new eruption caused the collapse of the Pomigliano d’Arco arcade. Three and a half kilometers of the aqueduct collapsed, interrupting the supply to all cities except Nola and Acerrae.

The poor economic situation of the time prevented the necessary repairs, and the cistern of Misenum was definitively taken out of use.
Thus, one of the largest water reservoirs of the Roman Empire gradually deteriorated over time, although it has survived to our days in relatively good condition.
After several interventions since the early 20th century, such as archaeological excavations and consolidation work on the walls and restoration, the Piscina Mirabilis is open to the public, with visits organized by the Archaeological Park of the Phlegraean Fields.
This article was first published on our Spanish Edition on February 15, 2024. Puedes leer la versión en español en Piscina Mirabilis, el impresionante depósito de agua romano tan grande como una basílica
Sources
Piscina Mirabilis (Web Oficial) | G. De Feo, S. De Gisi, C. Malvano, O. De Biase, The greatest water reservoirs in the ancient Roman world and the “Piscina Mirabilis” in Misenum. Water Supply 1 July 2010; 10 (3): 350–358. doi.org/10.2166/ws.2010.106 | Wayne F. Lorenz, Giacinto Libertini, et al., Prominent features of the Augustan aqueduct in the Naples bay area | La Piscina Mirabilis, la più grande cisterna dell’antichità (Napoli Turistica) | Wikipedia
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