1.The reconstructed Mithraeum sited beneath Bloomberg's European headquarters.

London's first men's club

'We should remember this was just a cult for men. They all met underground, they were quite steamed up from the drinking that went on, and naked some of the time. One imagines the cult was, dare I say, quite a pungent experience,' said historian Bettany Hughes describing the cult of Mithras at the opening of the Walbrook Mithraeum in London.

The remains of the city's only known Roman temple have returned to their original site beside the vanished River Walbrook, seven metres below the new Bloomberg European headquarters designed by architects Foster+Partners. Here, it evokes the atmosphere of the cult of Mithras practised in underground temples that served a creation myth in which the god slayed a bull in a cave.

2. People queue to see the newly revealed temple in 1954.

Mithraism was Romano-Persian in origin. It had first attracted the troops of Alexander the Great as they headed towards India. Mithras is sometimes seen dining with the sun-god, Sol, and banqueting was part of the ritual practised by the celebrants, an all-male club of the military, merchants and even slaves. Rather like Freemasons, the initiates were called syndexioi, 'united by the handshake'.

'Sacrifice was almost certainly part of the ritual,' says Sophie Jackson, the leading archaeologist consultant from the Museum of London Archaeology (MOLA) which worked with the City of London on the 10-year project privately funded by Bloomberg. 'But live bulls are unlikely to have been killed in such a confined space.'

3. Excavating the Bloomberg site in 2012. © MOLA.

The Walbrook Mithraeum, dating from around AD 240, was first revealed after wartime bomb damage was being cleared away in 1954. It proved a sensation, and a clamour from the press and public, who arrived to see it at the rate of around 30,000 a day, saved it from oblivion. However, to allow planned building work to go ahead, it had to be moved 100 yards away, where it became rather forgotten.

Visitors enter the new site through Bloomberg SPACE, a ground-floor art gallery that will exhibit a series of commissions from artists. The first is a colourful tapestry and a tubular metal sculpture by the Irish artist Isabel Nolan.

The gallery also contains a large glass cabinet where 600 artefacts, the pick of the haul of 14,000 excavated from the River Walbrook are displayed. These included more than 400 waxed writing tablets with the names of the early Londoners, and the first written reference to 'Londinium'.

4. Leaping bull plaque, the astrological sign of Taurus, found at the site. Lead alloy. L. 72mm. © MOLA.

The darkened mezzanine floor gives a flavour of the rituals and beliefs of the cult, the temple and its contents. Finally, visitors reach the 8m x 18m Mithraeum, laid out before a neon outline of the image of the tauricidal god in a 'son et lumière' setting created by Local Projects, with music played on replica Roman instruments and chants taken from graffiti found at the site. Excavated Roman mud was hand-painted to recreate the floor, and the original mortar was matched to rebuild the 2m-high perimeter wall. Street noises from Roman Britain fade into chanting as lights rise through the mists of history to reveal the ancient stones and paving. In the search for authenticity, says Jackson, 'No stone was left unturned'.

Introducing the project, the philanthropic Michael Bloomberg said he wished he could have done so in Latin, but it hadn't been on the curriculum of his Boston school. 'London has a long history as a crossroads for culture and business,' he said, 'and we are building on that tradition. As stewards of this ancient site and its artefacts, we have a responsibility to preserve and share its history.'

• Visit www.londonmithraeum.com for further details of the London Mithraeum or to book a free visit.
Roger Williams

Dominic Green