1. Aerial view of Lake Nemi with the Museo delle Navi, which houses scaled-down replicas of Caligula's pleasure-boats and other artefacts, on its shore.

The hidden secrets of Lake Nemi

Headlines last spring announced that a third pleasure-boat built for the Emperor Caligula (r AD 37–41) was about to be recovered from the murky waters of Lake Nemi near Rome. As it turned out, after investigating the facts with the former director of the Museo delle Navi at Nemi, archaeologist Giuseppina Ghini, this was not entirely a matter of 'much ado about nothing'. Huge floating palaces were indeed built and used for the entertainment of the decadent emperor Caligula and two of these (measuring 71.30m x 20m and 73m x 24m) had been recovered from the bottom of Lake Nemi, thanks to a brilliant rescue operation in the early 1930s.

Unfortunately those ships were destroyed either by American bombs or vindictive German soldiers who set fire to them in 1944 during the Second World War. Some smaller scale models of the original hulls, and objects salvaged from pillaging and wanton destruction, were later reassembled and re-housed in the specially built Museo delle Navi, on the shores of the circular volcanic lake.

A third imperial ship was, however, believed to be still lying on the bottom of the lake, and this was the subject of the recent hullabaloo. In the event only a small (8m x 2.5m) boat, which might have been used when the first wrecks were lifted out of the waters last century, was located. At the time the water level of the lake (which has no natural outlets) was lowered by 12 metres by complex siphoning and water redistribution. Analysis of the wood from this small boat is part of a current project undertaken this year that primarily concerns checking the pollution levels in the lake water, as well as using a side scan sonar Klein System 3000 to check for the presence of objects on the bottom of the lake.

2. Medusa head protome, bronze, 1st century AD, found in Lake Nemi.

The technical data collected so far has been transmitted to the National Institute for Naval Architecture (INSEAN), a research institute in Rome active in the field of naval architecture and marine engineering, which also tests the large modern Italian boats that take part in the international race for the America's Cup.

Caligula's 1st-century lake boats were not just used to pander to the eccentric and decadent tastes of a famously psychotic emperor, they were also used during rituals originating in the nearby temple of Diana, now currently under excavation by a team led by Giuseppina Ghini. This was a very ancient Latin shrine devoted to the cult of Diana Nemorensis (Diana of the Grove) on the northern shores of the crater-shaped lake, which is also known as Speculum Dianae (Diana's Mirror). Vitruvius described the temple as archaic/Etruscan in style. Before it there was a sacred grove where there stood a triple cult image of the goddess representing her as the virgin goddess of the hunt, of childbirth, of the moon goddess, and of the nether world, as Hecate. This image was recorded on Late Republican period coins. From its archaic beginnings during the Bronze Age, the shrine grew into a grand complex including a bath and a theatre that attracted crowds of pilgrims and the sick; it survived
into the 2nd century AD.

The social anthropologist James Frazer (1854–1941) believed it was here that a bloody fertility rite, entailing the murder of the temple's priest king, the Rex Nemorensis, took place. This ritual killing inspired his influential book, The Golden Bough, a comparative study of mythology and religion.

3. Visitors in the 1930s queue to see the second of Caligula's pleasure-boats.

A sacred tree stood in the grove. No one was allowed to break off any of its branches, with the exception of a runaway slave who, if he could manage to do so, would then challenge the priest-king in mortal combat and try to kill him in order to take his place, until he too was challenged by a newcomer.

Caligula's ship would actually have been used for the annual Isidis Navigium (The Ship of Isis), a spring, carnival-like festival in honour of the goddess Isis enacted at Nemi. Both genuine Egyptian and Egyptian-inspired objects pertaining to the cult of Isis and Bubastis were found in the original boats and at the temple of Diana. But by the time Caligula staged the Isiac rituals at Nemi the murder-succession of the priest-kings had evolved into a theatrical event, possibly involving a gladiatorial combat before an audience.

A trail along the shore of the lake leads to the entrance of the famous Emissary, a 1600-m long tunnel dug into the rock in the 5th century BC. The culvert regulated the water level of the lake, which was used to irrigate the surrounding valley. This masterpiece of ancient hydraulic engineering can be explored and, although less exciting than finding a royal barge with marble floors and columns studded with jewels where orgies took place, this tunnel is a truly awe-inspiring structure.

The search for long-lost archaic and imperial artefacts continues in and around the lake. In 2011, the Italian tax-police retrieved, from a smuggler's truck near Nemi, a large (2.50-m high), 1st-century AD marble statue, probably depicting Caligula; it is now the centrepiece of the lakeside museum.

• Museo delle Navi Romane, Nemi (www.museonaviromane.it).
Dalu Jones