Some 3,400 years before the roaring torrent of the A303 road sliced the Stonehenge landscape in half, some people cut a beautiful pit a metre deep into the chalk with no tools except picks made of red deer antlers.



They may have had primitive tools, but there was nothing primitive about their skills: the bottom of the pit was so neatly levelled that you could balance a beaker of mead on it without spilling a drop.

As druids and tourists head towards Stonehenge for the winter solstice, which falls this year on 22 December, when the midwinter sun should set framed perfectly by the giant stones, Historic England archaeologists are hard at work teasing ancient secrets out of the landscape.



The newly discovered pit was immaculately cut to hold a huge wooden post. A neat trench links to a second equally impressive pit for another massive post: in the rolling chalk downland, they would have been visible for miles. The line of the trench seems to lead on towards the neighbouring field full of curious Waitrose pigs, under a later bank but carefully jinking to avoid an earlier long barrow.



But what is it? Phil McMahon, Historic England archaeologist, and his opposite number at the National Trust, Nick Snashall, laughed and shrugged.



“A gateway? A boundary marker? A palisade? The truth is we just don’t know,” Snashall said. “We won’t even have a date [that it was created] until we get the lab results back.”



“This is keyhole surgery,” McMahon said. “We’re throwing up as many questions as we answer.”



Their survey – including aerial photography, ground penetrating radar, the study of centuries of maps showing now-vanished monuments, and excavations yards from winter crops and the pigs – is assessing the presumed route of a major intervention planned in the landscape, a tunnel to replace the present road. After decades of argument, in which traffic on one of the main arteries to the west has increased to the point that the road frequently becomes a fume-belching linear car park, replacing the road with a tunnel to the south of the present line has been included in the government’s five-year roads plan.



Tourists very rarely venture across the road – at busy times it is almost impossible on foot – but thousands of monuments, recorded and still to be discovered, including round and long barrows, linear monuments, burial mounds and ring shaped banks which once surrounded ponds, lie among the fields and clumps of woodland.



Many of the known sites have never been excavated, and the survey has also revealed some new ones, including two burials spots which may date from the iron age – laboratory work is continuing on all the organic finds – and a puzzling square enclosure which could be prehistoric, Roman or medieval, almost on the shoulder of the road. But the survey has demolished the claims of other sites. One presumed burial mound, which appeared from the air to have an exciting surrounding circle of pits for burials or ritual deposits, proved to be a medieval dew pond. The survey has also shown how intensively the landscape was farmed for thousands of years: one long barrow was completely ploughed out above ground by the time the Romans arrived.



Duncan Wilson, chief executive of Historic England, believes that this time the will is really there to solve the road problem: “I know my predecessors have said this before, but I do believe this time it is going to happen. It may be a bit of a cliche, but I really do believe we now have a once in a generation chance to reunite the landscape setting of one of the world’s most famous ancient monuments. The re-sited visitor centre has already made a major contribution, but the road was the great outstanding problem.

“There are still many questions to resolve about the details of the tunnel and where the portals should be sited, but I think the advantages of a tunnel of at least 2.9km, which is what the government is proposing, far outweighs the disadvantages.”

While broad agreement has been reached between Historic England, the statutory authority for ancient monuments, the newly split off English Heritage, which has guardianship of Stonehenge itself and the new visitor centre, and the National Trust, which owns thousands of surrounding acres of land, nothing happens at Stonehenge without passionate argument. Every move the archaeologists make is watched by the Stonehenge Alliance – a group that includes local residents, landowners, historians, druids and the Campaign to Protect Rural England – who argue for a much longer tunnel with the entrance and exit placed well outside the world heritage site (WHS). The alliance believes that doing nothing about the road would be much better than doing the wrong thing.

Kate Fielden, an archaeologist and member of the alliance, said: “The Stonehenge Alliance now has almost 20,000 signatures asking for no further damage to the world heritage site.

“It is extraordinary that archaeological evaluation trenching should be taking place within the WHS before any final decision on tunnel length, engineering feasibility and costing have been investigated. Our bottom line is that there should be no further damage to the WHS.”

McMahon and Snashall say the design of the tunnel, the portals and problem areas such as the link roads and the lighting of its approaches, must be “exemplary”.

“We can’t just let the highway engineers reach for off-the-shelf solutions – but if we get this right there is a chance of doing something which finally rejoins the two halves of this amazing landscape, and which can also set the standard for other ancient sites across the world.”