Piscina Mirabilis

It was built under Augustus (as suggested by the building technique of opus reticulatum used in the walls and pillars), in Bauli, the name that the ancient Romans gave to the current city of Bacoli,[3] exactly on the promontory between the historic centre and Punta Pennata which once surmounted the ancient port of Miseno (Misenum).[2]

It is thought to be situated there in order to provide (drinking) water to the Roman western imperial naval fleet with its battle ships belonging to the Classis Misenensis, once moored in the natural port of Misenum[3] (having relocating from the nearby Portus Julius in Puteoli, present-day Pozzuoli, because of silting problems), which at one time saw Pliny the Elder in command (who from Miseno, in 79 AD, recounted the eruption of Vesuvius),[2][3][4] but this is unlikely^(contested) as the cistern is about 1 km away from the slopes of the promontory of Misenum where the military base and residential area port were located.^(contested) Also, from the Augustan period the naval base was directly connected to the main Roman aqueduct, the Aqua Augusta, which was the largest aqueduct system in the Roman world,[5] and did not need the cistern.^(contested) More likely^(contested) is that the cistern belonged to one of the many luxurious villas built in this area, like the nearby Grotta della Dragonara[6] [7] cistern, which is dug into the slopes of the promontory.

The Grotta della Dragonara, an Ancient Roman cistern, with water that flows from the subsoil due to the presence of a thermal spring, is believed to have possibly served a vast residential complex, a villa maritima (seaside villa), located a little further south and visible on the ridge of the promontory. The villa, arranged in terraces with rooms that sloped down to the sea, is attributed by literary tradition to have once belonged to Licinius Lucullus (where, according to the historian Tacitus, the Emperor Tiberius died under suspect circumstances), from which the annexed fish ponds (the so-called Peschiere di Lucullus) sloped down into the sea.[7]

The Piscina Mirabilis cistern was supplied with water from the Aqua Augusta, which brought water to most of the sites around Naples and Campi Flegrei (Phlegraean Fields), up to the 8 meters above sea level of the Piscina Mirabilis (now at an altitude of 2 meters, due to bradyseism), from sources in Serino near Avellino, located on the Irpinia plateau at an altitude of 330[3] or 376[8][9] ^{(more likely but to be confirmed)} meters above sea level, about 100 kilometres away.[1][3]

Water was pumped out of the cistern using machines placed on the roof terrace of the cistern, which were extended in the 1st century AD by adding a series of 12 supporting barrel-vaulted rooms on the north-west side.[10]

The Augustan aqueduct was destroyed between the 4th and 5th century AD, which meant the Piscina Mirabilis could no longer be of use and therefore it is, to this day, emptied of water.[3]

The piscina was qualified with the adjective Mirabilis by the great 14th C. Tuscan poet Francesco Petrarca on one of his visits[11][12] for the extraordinary dimensions of its hydraulic structure which, entirely hewn out of a tuff hill at a depth of over 15 meters, make it similar to a real underground monolithic church.

Ichnography (floor plan) of the Piscina Mirabilis, a Roman Cistern located in Bacoli (IT), near the ancient Roman port of Misenum. Th. Rajola Arch, de. & Fiorillo, Sculp. (Tab. LXII), 1768

Testament to its monumentality are the dimensions: 15 metres (49 ft) high, 72 metres (236 ft) long, and 25 metres (82 ft) wide. The capacity is 12,600 cubic metres (440,000 cu ft),[1] amounting, in other words, to 12.6 million litres (3.33 million US gallons) of water, or, roughly the size of 5 Olympic-size swimming pools.

It was built as a kind of hypostyle hall[10] on a quadrangular plan to obtain four rows of twelve cruciform pillars per row which divide the interior space into five long naves and thirteen courtyards (just as if it were a cathedral, hence its local nicknames of "the Water Cathedral"[3] or the "Cathedral of Bacoli"[2]). The 48 columns support a barrel vaulted ceiling covered by a roof terrace made of Roman concrete (opus caementicium) and paved in opus signinum ('cocciopesto' in modern Italian) — a water- and damp-proof mix of terracotta fragments and lime — while gaining access to the interior is made possible by several doors and traps.

The piscina had two entrances (AA), a staircase supported by three arches[11] in the north-west corner and one in the south-east, currently closed. The only one of the two staircases (CC) that still allows access to the main nave is the north-western staircase thanks to a metal staircase with handrail (useful to preserve the ancient staircase from the trampling by visitors). Given the absence of holes visible from the outside, it is presumably so that the water was introduced through pipes coming from the North-West entrance (D) while it was extracted from above through ancient hydraulic systems, exploiting the wells of the barrel vaults through which the sun's rays today draw suggestive and fascinating plays of light and colours, called the Tyndall effect.

Legend: "Tab.(Plate) LXII. Plan of the admirable pool. There are two entrances AA. The two steps CC lead one down to the lowest floor. The floor is lowered even more in BB to allow the collection of water during purging. From this place the height is 39 palms[13]. Letter D. shows where the aqueduct stood."

The walls and pillars of the pool are in opus reticolatum, the characteristic Roman building technique of the Augustan era, with recourse to bricks for the walls and to tufelli [14] for the pillars. The latter are waterproofed thanks to a layer of cocciopesto and the technique of smoothing the corners through curbs placed at their bases. In the middle of the short central nave there is a 1.10 meter deep basin (BB), hollowed out in the floor and provided with an outlet at one end[15], which served as a so-called piscina limaria (waste-bath, i.e. a settling and drainage basin[15]), useful for the decantation, cleaning and periodic emptying of the cistern.

Leaning against the outside of the north-eastern side of the Piscina Mirabilis are 12 additional small rooms, also with barrel vaults and opus reticulatum walls, with the floor level 1.80 m lower than the impost of the cistern vault.[15] These rooms, constructed in opus mixtum and opus listatum[16], represent an upgrade of the hydraulic system[15] and were probably used for the storage of tools and hydraulic equipment. Dating back to a later construction, relating to the end of the 1st C. AD (and beginning of the 2nd C. AD[15]), in one of them is visible a cocciopesto floor with labyrinth-shaped mosaic tesserae and a central white inlaid panel with limestone polychrome tiles, which seems to date back to a more ancient phase.[5]

Tyndall effect in the Piscina Mirabilis

For the construction of the cistern, Roman engineers used to, at that time, mix flakes of raw tuff (extracted from the Neapolitan Yellow Tuff, produced by an eruption in the Phlegraean Fields of about 15,000 years ago), fresh water of hydrated lime and cocciopesto, the latter characterized by a mixture of lime, pozzolan and a part formed by broken-up tiles, bricks and crushed ceramics, able to give hydraulicity to lime. Extremely interesting is the composition of the adhesive cement mix, which results from the reaction of lime, cocciopesto, calcite, gypsum and tobermorite.

Engraving of a drawing of the Piscina Mirabilis, a Roman Cistern located in Bacoli (IT), near the ancient Roman port of Misenum. Signed: Joan. Baptista Natali del. & Joan. Volpato, scul. Venetiis (Tab. LXI)

A first restoration took place in the first half of the last century and focused more on consolidating some tuff pillars and vaults, and filling some cracks. The first work documented by the archives of the Soprintendenza is the completion of the excavations of the monument between 1910 and 1926, followed by the consolidation of the damaged walls. In 1926 a restoration of the damage of the second and third supporting arch was carried out at the time of the first nave, the surfaces of the pillars were restored with mortar lime and local pozzolan and the walls with new insertions of opus reticulatum. In 1929 work was carried out on the access staircase covered with a layer of cement and cocciopesto, 3 cm thick. In 1936 the consolidation work for the damaged arches continued with the reconstruction of the missing parts of the vaults, then repairing the extrados with cement and concrete. After this restoration, only in 2007 a new project was implemented to consolidate and waterproof the roof terrace of the monument.

The ancient cistern is in private hands, but it is open to the public,[4] except on Mondays, upon reservation, since 2020, due to a public-private partnership between The Archaeological Park of the Campi Flegrei and the ATS (Temporary Association) StraMirabilis.[3]

• List of Roman cisterns
• Basilica Cistern
• Cistern of Philoxenos
• Theodosius Cistern

• Paoli, Paolo Antonio. (1768). Antiquitatum Puteolis Cumis Baiis Existentium Reliquiae. Avanzi delle antichità esistenti a Pozzuoli, Cuma e Baja. Napoli: [s.n.], Anno A.C.N.MDCCLXVIII ^{(with full book scan available at German Archaeological Institute (iDAI) )}, a.o. texts related to Expl. Fol. 34 & Tab. LXI[17]
• 

  Explanation to book plate Tab. LXI - Interior view of Piscina Mirabilis, Expl. Fol. 34, Paolo Antonio Paoli, 1768 (click on image to access translation in English via "More Details" (bottom right) on Wikipedia Commons)
  De Feo, Giovanni & De Gisi, Sabino & Malvano, Carmela & De Biase, O. (2010). The Greatest Water Reservoirs in the Ancient Roman World and the “Piscina Mirabilis” in Misenum. Water Science & Technology: Water Supply. vol. 10, issue 3, pp 350–358. Publication by IWA Publishing, 2010.

• Official site Piscina Mirabilis - Bacoli
• Official Campania Tourism Site
• Official Tourism Portal - Comune di Bacoli (Italian)
• Archaeological Park of the Phlegraean Fields - Parco Archeologico Campi Flegrei (Italian)