It is the M4 motorway, which for over 20 years has acted as a natural limit to the spread southwards of one big town, Reading in Berkshire, towards another one 15 miles away, Basingstoke in Hampshire.
It is a barrier that has held back some of the severest pressure for development in all of Britain. Now it is about to be breached: a substantial new settlement of 2,500 houses is being planned in green fields south of the motorway.
Local campaigners fear that this will open the gates to urbanisation of the whole stretch of picturesque countryside between the two towns, a vision of awfulness they have termed Readingstoke.
Thirty-two parish councils have joined together with the Council for the Protection of Rural England (CPRE) to fight the proposals, which they say make a nonsense of the Government's new-found commitment to allow house building on greenfield sites only as a last resort.
The inquiry, which may last a year or more, will indeed be seen as a vital test case of the willingness of the Environment Secretary, John Prescott, to back up with action his words last February that "it is our firm policy to protect our countryside ... by maximising the use of recycled land."
Mr Prescott will himself take the ultimate decision as to whether or not the M4 barrier should fall.
The motorway has been treated by Berkshire County Council planners as a sensitive and natural boundary to the countryside ever since it was built in the 1970s. Successive proposals for large-scale housing development beyond the M4 have hitherto all been rejected.
But the M4 is also the problem: it is the artery of England's "Silicon Valley", the band of development of hi-tech industries along its length from west London almost to Bristol, that have brought jobs and prosperity to the area - and a huge demand for housing.
Berkshire as a result has suffered the severest development pressure of any English county, with dire results.
"It's always been the place that everyone else didn't want to copy," said Tony Burton, the CPRE's assistant director for policy. "It's been asked to accept very, very high levels of development for 20 to 30 years, and so the whole character of the county has changed out of all recognition.
The loss of the identity of Berkshire's settlements, the blurring and merging together of Wokingham, Bracknell and Reading, has turned what was countryside into a world of suburban estates and mini-roundabouts."
Until now, southern Berkshire has been immune and remained green and properly rural, thanks to its M4 defence line. But housing pressure finally spilled over it with the drawing up of the Berkshire County Structure Plan in 1995, when the then Conservative Environment Secretary, John Gummer, ordered the county council to accept 40,000 new houses instead of the 37,000 it firmly believed was the the maximum that could be built without serious environmental damage. The extra 3,000 houses, the council warned, would have to go south of the M4, but Mr Gummer insisted anyway.
Wokingham District Council is now the planning authority charged with locating them, and its preferred site for 2,500 centres on the hamlet of Grazeley, a tiny village sitting in the fields with a pub, a church and a village hall.
If the development by a consortium of Persimmon, Taylor Woodrow and McAlpine goes ahead, Grazeley will be expanded a hundredfold over the surrounding countryside.
Two further sites in the area have been proposed by other consortia, at Spencers Wood and Shinfield, and the inquiry will examine their competing claims. The district council takes the view that this is all it can do, and that the principle of development south of the M4 has already been established.
But the campaigners disagree and are preparing an energetic challenge. "This inquiry has important national implications," said Edward Dawson, who is a committee member of the action group. "It is really testing whether the greenfield sites policy, that Mr Prescott announced in February, works or not.
"We all have a nightmare vision of Readingstoke, of Reading joining up with Basingstoke and the countryside in between being destroyed. "The campaigners are planning to put forward an alternative brownfield site for the development, north of the motorway.
On Friday the local MP, John Redwood, the Shadow Trade and Industry Secretary, was applauded by Grazeley's villagers when he planted an oak tree on the village green to symbolise their determination to fight.
"People have had enough of seeing the green fields of Berkshire built over," he said.