By Jemima Laing
The site is at Tottiford reservoir near Hennock
When the water levels at a Devon reservoir were lowered it revealed an unexpected prehistoric surprise.
A previously unrecorded complex was discovered in the mud at the bottom of Tottiford Reservoir near Hennock on Dartmoor.
The complex, which is believed to be 4,000 years old, is made up of stone rows, burial cairns and a stone circle.
The discovery is being described as one of the most important on Dartmoor in recent times.
The area has been surveyed in detail
It had been hidden by the waters of the reservoir - the first one to be built on Dartmoor in 1861 - and was only revealed because water levels were temporarily lowered by South West Water to just 6% capacity.
Mike Miller from nearby Moretonhampstead spotted the significance of the site and contacted Jane Marchand, an archaeologist with Dartmoor National Park Authority.
He told her he had found two stone rows and some burial cairns.
Further visits to Tottiford confirmed Mike's findings and also led to the discovery of a large stone circle, measuring 22 metres in diameter.
"If you look at the stones they're all facing into the circle," said Jane.
"They've all been deliberately put so they're sideways on and I think that's actually what decided us it was definitely a stone circle."
She said the location of the site - on the most eastern spur of Dartmoor - was "particularly interesting".
"This is an area with little recorded prehistoric archaeology compared to the rest of the moor," Jane explained.
Some finely worked flint tools were also discovered
"There was no knowledge of the existence of this complex as the reservoir's construction pre-dates the beginning of systematic archaeological recording on Dartmoor."
The entire ceremonial complex consists of a free standing stone circle, a double stone row and single stone row, with regularly spaced stones and which both seem to end on burial cairns.
Kevin Jones from South West Water said the reservoir level has been even lower before and he was surprised no one had noticed the site before.
"But I think it's been low for so long that the mud's dried up a little bit and it's probably exposed it, which is quite exciting for everybody."
As well as the ceremonial site itself some finely worked flint tools were also found close to the complex.
The site will eventually disappear under the water again
These include knives, piercers, notched blades, microliths (very small flint tools used as barbs, tips of arrows, or placed edge-to-edge in a wooden haft) and some cores which are the remains of the nodule of flint from which the tools were made.
"Some of the tools date back to Mesolithic times 8,000 years ago indicating that this area has been the focus of human activity over many thousands of years," said Jane.
The area has been surveyed in detail, and some preliminary geophysical survey work carried out before the site disappears beneath the water once more.