A major archaeological investigation is getting under way at one of Western Europe's most impressive prehistoric sites.
The Ring of Brodgar in Orkney is the third largest stone circle in the British Isles, but little is known about it.
A month-long programme of investigations will be undertaken by a 15-strong team.
The last important archaeological studies took place there in the 1970s.
Significant developments have taken place since then in analytical techniques including dating.
Historic Scotland said very little was actually known about the site, including its exact age and purpose.
A scheduled ancient monument, the stone circle and henge of the Ring of Brodgar is part of 'The Heart of Neolithic Orkney' World Heritage Site, inscribed by UNESCO in 1999.
The project will involve the re-excavation and extension of trenches dug in 1973. Geophysical surveys will also be undertaken to investigate the location of standing stones.
Dr Jane Downes of the Archaeology Department, Orkney College, UHI, and Dr Colin Richards of the University of Manchester are the project directors.
Dr Downes said: "Because so little is known about the Ring of Brodgar, a series of assumptions have taken the place of archaeological data.
"The interpretation of what is arguably the most spectacular stone circle in Scotland is therefore incomplete and unclear."
Dr Richards added: "At present, even the number of stones in the original circle is uncertain.
"The position of at least 40 can be identified but there are spaces for 20 more."
The project is being partly-funded by Historic Scotland.
The Ring of Brodgar is situated on a low-lying piece of land, separating the lochs of Harray and Stenness.
Other features in the area include the Stones of Stenness, a much smaller stone circle set within an enclosing ditch, Maeshowe, a Neolithic chambered cairn and a number of standing stones.
Unlike the Ring of Brodgar, most of the other monuments within this group have been examined by modern digs.
This latest dig also includes archaeologists and scientists from Stirling University and the Scottish Universities Environment Reactor Centre.