A MYSTERIOUS stone structure has been uncovered at one of Scotland's most famous ring of standing stones.

Archaeologists digging at the Ring on Brodgar on Orkney say that the layout of a series of newly-found slabs is unlike anything previously found on the islands.

The slabs, known as orthostats, were unearthed on the last day of this year's excavations of a complex of Neolithic buildings in the area.

They were buried underneath what's been dubbed Scotland's "largest Neolithic rubbish dump", and would have been part of a structure measuring almost 10m (33ft) wide, comprised of slabs of up to 4m (13ft) in length.

Experts say that the find could be the oldest of the buildings under covered so far, and might be be 5,000 years old.

However, they are mystified as to why the structure was covered over by the huge midden, and have speculated that it could possibly be a chambered tomb.

The dig team, which is led by University of the Highlands and Islands Archaeology Institute, said further "hard work" would be needed to properly understand the find.

Since 2002, Neolithic buildings, artwork, pottery, animal bones and stone tools have been discovered at Ness of Brodgar, the location of the Ring of Brodgar standing stones.

Site director Nick Card said: "The sheer size and scale of the stones unearthed are unprecedented on this site.

"The way the stones are built into the construction is also unique to the Ness. This all suggests that they may have been re-used and taken from elsewhere.

"Perhaps they may be part of a stone circle that pre-dates the main Ness site. It is all a bit of mystery and we won't know more until we do more work."

Last month, a human arm bone was uncovered during the dig.

Archaeologists believe the bone was deliberately placed and could possibly be the remains of a respected original founder of the large complex.