A special radar machine is being used to survey Edinburgh's historic Royal Mile in a bid to find out what lies beneath the famous thoroughfare.
Archaeologists hope to find the foundations of the Tollbooth
Archaeologists hope to find the foundations of Edinburgh's 15th century jail, known as the Tollbooth.
Although experts know it was situated near St Giles Cathedral, they hope to pinpoint the exact location during a series of surveys over the next week.
The project is being carried out during work on the area's road surface.
Edinburgh City Council archaeologist John Lawson said work on the setts would provide an opportunity to identify exactly where the important building lay.
"It's known that the foundations of Edinburgh's Tollbooth are on the Royal Mile, near St Giles Cathedral.
"Plans from the late 17th and early 18th century plot their location, but we're excited to be able to pinpoint them exactly.
"Built in late medieval times, the buildings were demolished in the early 19th century. In their heyday, they played an extremely important role in the city, as a town hall, Edinburgh's Tollbooth and the town jail.
"This work will allow us to accurately interpret what remains for both residents and visitors to the Royal Mile."
Further archaeological work will follow the radar surveys.
Work on the reconstruction of the road surface between George IV and North Bridge began last month.
The £1.5m reconstruction project is being undertaken to prevent further damage to the road and to avoid any future need for unplanned emergency repairs. The work will be completed by early 2007.
The reconstruction will involve re-laying thousands of existing traditional granite cobbles.
The existing granite setts will be used to maintain the historic appearance of this section of the Royal Mile and they will be laid in a traditional Edinburgh pattern, as used elsewhere in the city.
Historic Scotland, Edinburgh World Heritage Trust and Conservation Planning have all approved the design.
Councillor Andrew Burns, Edinburgh City Council's transport leader, said: "These crucial works, to restore the cobbles of the Royal Mile to their rightful glory, are providing a fantastic opportunity to learn more about the history of our city."