A rare marble inscription referring to a fulgur submanium, a nocturnal lightning strike attributed to Summanus, has been discovered at Hadrian’s Villa in Tivoli. The find emerged during archaeological investigations by the Department of Humanities (DISTUM) of the University of Urbino Carlo Bo, led by archaeologist Fabio Giorgio Cavallero, in collaboration with the Istituto Villa Adriana e Villa d’Este.

The team made the discovery in a sector between the so-called Amphitheatre and a natural bank of tufa. Here, the archaeologists uncovered a small quadrangular structure made of tufa elements bound with mortar. Set into its front is a marble slab measuring 40 × 35 cm, still preserved in its original position. The inscription, arranged in two lines of capital letters, reads FVLGVR SVBMANIVM. 

The inscription FVLGVR SVBMANIVM refers to a nocturnal lightning strike associated with Summanus, the Roman god of thunder and lightning by night. Pliny the Elder (Natural History 2.138) distinguishes between lightning during the day and at night, attributing the former to Jupiter and the latter to Summanus. Summanus was an ancient Roman deity of obscure origin. A temple (Aedes Summani) dedicated to him stood near the Circus Maximus and was traditionally said to have been built during the war with Pyrrhus in the early 3rd century BC. His festival, the Summanalia, was celebrated on 20 June. Ancient sources also relate that a terracotta statue of Summanus which stood on the Temple of Jupiter Optimus Maximus was struck by lightning, and that its head was later found in the River Tiber.

For the Romans, a place struck by lightning acquired special religious status. The ground was considered religiosus and set apart from ordinary use. Objects and materials affected by lightning were gathered and buried, accompanied by the appropriate rites and sacrifices. This ritual was known as condere fulgur, literally “to bury” or “lay away” the lightning, while the deposit itself was called a fulgur conditum. The ritual was closely connected to Etruscan religious practice. Interpreting lightning was an important part of the Etrusca disciplina. It was treated in the libri fulgurales, the sacred books that dealt with the different types of lightning and their significance. Roman haruspices, drawing on this tradition, were called upon to interpret such signs and prescribe the appropriate rites of expiation. Pliny notes that Etruscan teachings recognised nine gods capable of sending eleven different types of lightning.

The sacred area associated with the strike was known as a bidental. People could enclose or mark out such places, and they were not to be disturbed. The name was traditionally connected with the sacrifice of a two-year-old sheep, or bidens, during the purification of the site. At Hadrian’s Villa, the small quadrangular structure into which the inscribed slab was set may have marked or delimited such a bidental. The inscriptions marking these places did not follow a single fixed formula. Some read Fulgur conditum, while others distinguished daytime lightning (Fulgur Dium conditum or Fulgur Divom conditum) from nocturnal lightning associated with Summanus, expressed in forms such as Fulgur Summanum conditum, Fulgur conditum Summanium or Fulgur Summani. The FVLGVR SVBMANIVM from Hadrian’s Villa stands out as another variant, as it gives the nocturnal classification but does not include conditum on the slab itself.

Other archaeological examples of this practice are known. One of the best known is the colossal gilded bronze statue of Hercules, discovered near the Theatre of Pompey in Rome in 1864. The statue had been laid horizontally in a pit and covered with travertine slabs inscribed with the letters F C S, generally interpreted as Fulgur Conditum Summanium. The statue had apparently been struck by lightning and was subsequently buried according to ritual practice, together with the remains of a lamb. Another parallel comes from Ostia Antica in the so-called Domus Fulminata (III, VII, 3–4), or House of the Thunderbolt. In the south-west corner of its courtyard is a small masonry bidental, where a marble plaque inscribed F D C, generally expanded as Fulgur Dium Conditum, marked the place where lightning had struck, and the affected objects had been ritually buried. Fragments of vessels, terracotta statuettes and lamps were reportedly found beneath the slab. The discovery gave the building its modern name, Domus Fulminata.

The discovery at Hadrian’s Villa is especially important because the inscribed slab is still in its original location within the structure to which it belongs. This structure may have defined a bidental, a sacred area connected to a site struck by lightning. The archaeological context of this finding also adds to its significance.

The so-called Amphitheatre lies at the foot of Piazza d’Oro, one of the most elaborate architectural complexes of Hadrian’s Villa. The monument is an elliptical structure whose original function remains uncertain. From the 16th century onwards, it was interpreted as a large basin, perhaps intended for bathing or for keeping fish. Cleaning of the area in the 1990s, however, revealed architectural elements that led to the suggestion that the structure may instead have been an amphitheatre. The identification has never been firmly established, hence the continued use of the name “so-called Amphitheatre”.

New archaeological investigations began here in 2024, when the Department of Humanities (DISTUM) of the University of Urbino Carlo Bo opened a stratigraphic excavation under the direction of Fabio Giorgio Cavallero. Just a few centimetres below the surface, the team uncovered the walls of a previously unknown building dating to the Hadrianic period. The building measured more than 25 metres long and 15 metres wide. It consisted of a large rectangular hall surrounded by eight rooms opening onto a terrace supported by pillars, some of which survive to more than three metres high. Fragments of white marble, green serpentine, and red porphyry, together with painted plaster in red, yellow, and green, revealed the richness of its decoration, featuring intertwined bands, acanthus leaves, and pomegranates. (see here)

At some point, part of this building was intentionally demolished to make way for the elliptical structure now known as the Amphitheatre. Instead of removing the debris, the builders utilised it to raise the foundation for the new structure’s floor. This practice explains why many fragments of marble revetment and painted plaster from the earlier building were found within the fill. The excavation revealed that the so-called Amphitheatre was not part of a single construction phase, as it replaced an earlier, richly decorated Hadrianic complex.

Investigations continued in 2025 as part of the University of Urbino excavation project. A campaign was conducted in September and October, although detailed results from that season have not yet been published. The work focused on exploring the various phases of construction and transformation in this little-known part of the Villa. It is within this increasingly complex archaeological context that the FVLGVR SVBMANIVM inscription was discovered.

Further excavation and stratigraphic analysis may eventually clarify the position of the fulgur conditum within the sequence of changes that affected this part of the Villa. (see here)