A Roman temple has been restored to its original site seven metres below the City of London, using sound, lights and misty haze to bring the ruin back to life.

Built in the third century, the London Mithraeum was discovered by chance in 1954 on a World War II bomb site. It became an instant public sensation, with up to 30,000 people per day queueing to see it.

The temple to the god Mithras was dismantled and reassembled 100 metres away from its original location so the public could see it when post-war rebuilding on the site was complete.

But now the ruins have been moved back and restored, deep beneath Bloomberg's vast new European headquarters by the Bank of England.

"London is a Roman city, yet there are few traces of its distant past that people can experience first-hand," said Sophie Jackson, the project's lead archaeological consultant.

The reconstruction puts the temple back in place, as it looked at the end of the 1954 excavation.

"This really was one of the most important discoveries in London, if not in Britain, in the 20th century," said Jackson, from the Museum of London Archaeology team.

The site is darkly lit and an audio soundscape reveals some of the noises and Latin chatter that visitors to the temple might have heard as Mithras cult members indulged in feasting, drinking and sacrificing.

Sheets of light from above, slicing through the misty haze, complete the temple walls.

- Mystery cult -

Mithras was a Roman deity often represented killing a bull, though it is widely believed the figure was derived from a Persian sun god.

His cult was all male, and the Mithraeum was a place for members to bond.

The temple was built next to the River Walbrook, a now submerged stream running through the City of London. The seam of damp earth helped preserve a wealth of artefacts.

Before the new Bloomberg HQ was built, a six-month dig uncovered the largest assemblage of Roman archeology ever excavated in London.

The dig turned up more than 15,000 objects including jewellery, shoes, animal bones and pottery, and brooches that shed light on where Roman Londoners came from, what they ate and how they lived.

It also unearthed a wooden tablet from January 8, 57 AD -- the earliest dated handwritten document from Britain.

Free to visit, the London Mithraeum opens to the public on Tuesday.