The mock battlefield, used for training soldiers before they were shipped across the Channel to confront the real thing, is complete with zig-zags of frontline, communication and reserve trenches, the enemy's front line, terrifyingly visible less than 200 yards away – and, a little further on, a holiday camp in Gosport, Hampshire.
Browndown is still owned by the Ministry of Defence, but well used by local dog walkers, who knew there were humps, bumps and hollows into which a dog could annoyingly vanish – but had no idea what they were.
Rob Harper, conservation officer at Gosport council, was originally studying the photograph looking for second world war pillboxes, and had to wait several weeks after he spotted the telltale marks – until the head-high bracken and gorse died back – before he could investigate the site.
He thought it was likely the earthworks had been destroyed since the photograph was taken, since Google Earth just showed a confusing jumble of tracks. But when he finally put on his boots and scrambled around the land, he found himself in a perfectly preserved complex covering acres of land.
The front trench was jagged so that even if the Germans broke through, they didn't get a clear line of fire along its entire length, and the communication trenches were wider so more men could be rushed up to the front – or carried back injured. Although very overgrown, the distinctive profile of the trenches is instantly familiar from countless wartime photographs.
"I was completely astonished at what I was seeing," Harper said. "It was quite personal to me too – I have seven relatives buried in war graves on the front, who could well have trained here."
The historian Dan Snow, who is also president of the Council for British Archaeology, which is working to record the site with English Heritage, said: "This is where archaeology and history dovetail perfectly. In a way this is where we have to side with Michael Gove and against the Blackadder view of history.
"This is where you can see on the ground that it wasn't just about rounding up young men and hurling them at the machine guns: they were being incredibly well trained."
Here military tacticians were also trying to invent a new form of warfare, desperate to break the terrible stalemate that the trenches represented. But according to Wayne Cocroft, an English Heritage expert on wartime archaeology, although 20 other trench training sites have been recorded across Britain, many have been damaged by later development, and both the scale and the state of preservation of the Gosport complex is exceptional.
So far no records have been found of the complex, but thousands of soldiers were trained, shipped out, and repatriated to Gosport throughout the war. The peninsula on the Solent is spattered with centuries of military relics, including barracks blocks, airstrips, naval bases, supply depots and a submarine base, which is now a museum.
Graham Burgess, deputy leader of the council – who graciously said that if the MoD would like to present them the land, after checking first for live ordnance, the council would be pleased to accept it – was not surprised, as an ex-navy man, that the site had kept its secrets for so long.
"Gosport was full of things happening behind high walls and barbed wire fences that nobody outside knew anything about – still has a few. You could live next door to one of these places and not have any idea what was going on inside."
Stephen Fisher, one of the archaeologists recording the site, says digging the trenches would also have been training for the men, who would soon have to do it for real, and the little slit trenches scattered across the site, just big enough for one man to cower in, might represent their first efforts.
Volunteers including armed forces members based in the area, including many for whom the site has a personal poignancy since they have just returned safely from active service overseas, will be helping record the site in detail.
The Council for British Archaeology and English Heritage are combining to encourage many more volunteers for the Home Front Legacy – a campaign to identify and record vulnerable sites including camps, drill halls and factories. The information will loaded onto a database to create a map of the social history of wartime Britain.
Cocroft said that records were better for a Tudor house than a 1914 site: one site recently identified, in Newcastle under Lyme, was a hall used as a sewing circle where women gathered to make bandages and knit and sew garments for soldiers on the front.
"There were so many charities for Belgian and Serbian refugees – where were they based, where did they meet? There were factories taken over to make things like wooden boxes for shells. These things aren't recorded on any maps – only local knowledge can help us find them, before the memory is lost forever. The Great War affected everybody in Britain – down to the children who were asked to gather conkers from which a chemical used in making cordite could be extracted – but there is so much of its social history which was never written down anywhere."
Culture secretary Maria Miller said she hoped local and family history groups, schools and parish centenary projects would get involved in the project: "Discovery, preserving and identifying for the public sites and buildings from that era will help bring that part of our national history alive for generations to come."