ARCHAEOLOGISTS are knee-deep at Lincoln Castle, unearthing evidence of a centuries-old prison buried deep underground.

The dig follows a geophysical survey carried out a few years ago when geophysicists used a resistivity meter and fluxgate gradiometer in an attempt to detect any buried walls or objects.

The investigation uncovered anomalies that archaeologists at the castle are now excavating.

Cecily Spall, 33, an archaeologist with FAS Heritage, who are working at the site, said they will be carrying out the exploration for at least another few weeks.

She said: "We have found two pieces of building already.

"The latest are positioned in the form of two trenches where stone walls once stood.

"We have to carefully take the trench down in separate layers. Every so often, we will come across a wall or floor surfaces.

"We have two of these walls within each trench, perpendicular to each other, and we think they both relate to the old jail.

"One theory is they converted medieval buildings already at the castle to be the jail."

It is also believed the Georgian prison building, still standing at the castle, was built at the same time the jail they have now discovered was demolished.

Documents no longer exist that confirm when the prison was built, but a map from 1783 shows a jail site on the north lawn where the dig is under way.

Anyone visiting the castle can ask the archaeologists questions and information boards are positioned near the site.

Ms Spall said: "It's nice to work in a place like this because we get people who are very interested and they interact with us."

The work itself marks something of a milestone in the castle's long history, dating back to 1068.

Angie Clay, 35, site co-ordinator at Lincoln Castle, said: "To the best of my knowledge, the castle has never had an archaeological dig before.

"We had a geophysical survey and it showed patches of what could be walls. It was just carried out on the main lawn and that basically has given us the area of the trenches.

"We have always known there was something here, but there's never been an opportunity to do this.

"We are not expecting to find vast amounts of materials. But any finds we do discover, there have been a few bits of pottery, will be with our conservators in our collections team."

Rod Callow, administrator of the Society for Lincolnshire History and Archaeology, said the dig was an incredible opportunity.

He said: "The work that is going on up at the castle is very significant. It is going to prove an important attraction to tourists in the future as well."