Stone monoliths found buried near Stonehenge could have been part of the largest Neolithic monument built in Britain, archaeologists believe.
The 4,500-year-old stones, some measuring 15ft (4.5m) in length, were discovered under 3ft of earth at Durrington Walls "superhenge".
The monument was on "an extraordinary scale" and unique, researchers said.
The Stonehenge Hidden Landscapes team has been creating an underground map of the area in a five-year project.
Remote sensing and geophysical imaging technology has been used to reveal evidence of nearly 100 stones without the need for excavation.
The monument is just under two miles (3km) from Stonehenge, Wiltshire, and is thought to have been a Neolithic ritual site.
Experts think it may have surrounded traces of springs and a dry valley leading into the River Avon.
Although no stones have been excavated they are believed to be fashioned from sarsen blocks found locally.
Sarsen stones are sandstone blocks found mainly on Salisbury Plain and the Marlborough Downs in Wiltshire.
A unique sarsen standing stone, The Cuckoo Stone, remains in the field next to Durrington Walls.
The stones are believed to have been deliberately toppled over the south-eastern edge of the bank of the circular enclosure before being incorporated into it.
Lead researcher Vince Gaffney, of the University of Bradford, said: "We don't think there's anything quite like this anywhere else in the world.
"This is completely new and the scale is extraordinary."
Archaeologist Nick Snashall said: "The presence of what appear to be stones, surrounding the site of one of the largest Neolithic settlements in Europe adds a whole new chapter to the Stonehenge story."
The earthwork enclosure at Durrington Walls was built about a century after the Stonehenge sarsen circle, but archaeologists believe the newly discovered stone row could have been put in place at the same time or even earlier.
Andy Rhind-Tutt, Heritage Trust founder, described the findings as "an incredible discovery".
He and University of Buckingham researchers have been involved in another nearby site, Blick Mead, thought to be at least 9,500 years old.
Mr Rhind-Tutt fears this and other sites could be damaged or lost to a planned A303 road tunnel past Stonehenge.
"It's a big concern to all of us, especially as we are at the tip of the iceberg with this particular discovery, and it would be horrible to destroy one of the most significant sites in the world," he said.
"The hidden treasure trove of the Stonehenge landscape just begs the question about why are all these incredible structures here?"
David Jacques, from the University of Buckingham, who is Blick Mead project manager, described the find as "absolutely brilliant "and a "game changer".
"All the monuments have a relationship with each other," he said.
"So rather than just 'atomising' them and looking at them as individual entities there are deliberate lines of sight or knowledge that things are just over the hill.
"When you put that together in the late Neolithic - there's something vibrant, exciting and dynamic [about the find]."
The findings were being announced on the first day of the British Science Festival being held at the University of Bradford.