It is an area of woodland that many people walk past without taking a second glance but the trees behind County Hall hide a secret.

Kim Briscoe

It is an area of woodland that many people walk past without taking a second glance but the trees behind County Hall hide a secret.

It might sound like something from the Da Vinci Code, but there is a stone archway buried in the undergrowth behind Norfolk County Council's headquarters, inscribed with mysterious letters.

Now, people are being invited to help reveal more about the building - and give wildlife a boost as well.

Marked on the map as a chapel, the archway, inscribed with the letters MGI or possibly MCI, is thought to be part of a folly in the grounds of a grand house that stood on the site off Martineau Lane before County Hall was built.

People say that the floor featured a mosaic pattern made from sheep knuckle bones and lumps of carved masonry are scattered throughout the woods.

Last month, volunteers removed encroaching vegetation to reveal the remains of another structure nearby and there has been speculation that this could have an ice house.

And now, people are being invited to join in the work to clear the site of overgrown holly to help improve biodiversity and find out more about the history of the area.

Mark Webster, from the British Trust for Conservation Volunteers' (BTVC) Norwich Green Gym, who is organising the event in conjunction with the county council, said: “We hope to discover more about the site's heritage, but also act to improve biodiversity by cutting back the densest patches of holly.

“There is lots of bluebells here that need a bit more light to survive, and it's even possible that the rare wild tulip - which hasn't been seen here for years - could make a comeback next spring.”

The work is part of the Green Gym scheme, which was set up in Norwich in June and is one of 90 in the country.

The project encourages people to get outside to get fit, enjoy nature and do something productive at the same time.

Mr Webster added: “The archway is well known but the fact that there are other remains is a bit of a mystery and we don't know what this other building is.

“It would be interesting if anyone had any memories of it or had any photographs.”

The group will be meting under the aeroplane by the County Record Office behind County Hall at 1pm on Monday, December 8.

Anyone who cannot make it but is interested in taking part in Green Gym initiatives, volunteers are welcome to take part in a wide range of other tasks every Sunday, Monday and Wednesday.

Anyone interested in joining in, should contact Mark Webster on 01603 767 300 or via www.btcv.org/norfolk

Are you part of an innovative conservation project or can you shed any light on the mysterious buildings? Contact environment reporter Kate Scotter on 01603 772326 or email kate.scotter@archant.co.uk