For residents of Centreville's Little Rocky Run community, the two trenches that cut through the woods behind their homes seemed like nothing more than dry ravines. Children pretending to be soldiers often used them as bunkers.

So when a builder last year proposed constructing 125 town houses on the 14-acre wooded lot, most neighboring homeowners didn't pay much attention, lamenting only that they were about to lose valuable green space.

Now, nearly a year after county officials rezoned the property, residents say they've learned that the trenches may be far more significant than they had realized and are hoping to curtail or scuttle the development.

The discovery also has prompted complaints from residents and historians who say county officials were remiss in not conducting a thorough survey that would have revealed the site's historical significance and would have made it easier to preserve it.

"We have uncovered what is of national significance for historical reasons," said Brendan Dillon, a resident whose home backs onto the wooded site. "The county may have missed it, but we aren't going to let this fall through the cracks."

Civil War historians and archaeologists say the two trenches--parallel to each other and running about 100 yards north to south--are remnants of Civil War fortifications associated with three key battles.

The larger trench, about 6 feet deep and 20 feet wide in some places, is believed to have been dug by Confederate soldiers to defend themselves against Union troops shortly after their victory in the First Battle of Manassas in the fall of 1861. It marked the farthest advance of Gen. Robert E. Lee's army into Northern Virginia.

A year later, as the tide of the Civil War changed, the area was overrun by Union soldiers who rebuilt the trench with berms and then dug a smaller one just west of it following the Second Battle of Manassas and then the Battle of Bristoe Station.

Frances H. Kennedy, author of "The Civil War Battlefield Guide," said the trenches tell a "bigger story" of the "push and take of the two great armies" in which the trenches were built one way and then turned around.

When the trenches were first dug, it was the "high point of the Confederacy," Kennedy said. "But things then went awry, and the Confederacy suffered tragic consequences."

As a result of their involvement in key Civil War battles, "the remaining Centreville earthworks are of national significance," said Edwin C. Bearss, chief historian emeritus with the National Park Service and noted Civil War expert, who examined the site.

Joseph L. Harsh, a history professor at George Mason University, called the trenches a "historical treasure of the first magnitude."

"Surely, someone, or some group, somewhere has the authority, the sensitivity and the vision . . . to save this small but hallowed plot," Harsh said.

But the effort to preserve the two trenches faces an uphill battle. Centex Homes, which owns the property, already has the rezoning for the town houses, and the county has no legal recourse.

And the trenches run through the entire center of the development, leaving little room for the developer to consider setting aside a portion of the land while still building the homes as planned.

Centex officials have told county officials they are sympathetic to the preservation efforts and have proposed setting aside a portion of the larger trench with a historical trail alongside it. Setting aside both trenches, however, would be costly because it would mean revamping the entire development, they said.

Centex officials could not be reached for comment.

This week, homeowners met with Centex officials and have proposed the possibility of purchasing the property if efforts to preserve the trenches fail.

Wade Elliott, a resident, said if Centex agrees to sell the property, the homeowners will appeal to the county and historical groups for funds. The purchase price may be as high as $3.4 million.

"Buying the property may be our only recourse," Elliott said.

Meanwhile, officials said that as a result of the controversy surrounding the trenches, the county has overhauled the way it examines properties that are being rezoned.

Residents contend that if an archaeological survey had been completed during the county's review of the rezoning application, the county could have demanded concessions from the developer, including preserving the trenches.

The trenches were uncovered after a resident, on a hunch, asked her brother, a Civil War buff, to examine them last November, more than five months after the development had been approved.

Prior to the Centex rezoning, the county's archaeology department, understaffed and lacking resources, was required to survey only about 40 percent of the properties under review for rezoning. Now it will be required to examine all properties.

"It's not something where you have any bad guys," said Michael Johnson, a county archaeologist. "Centex is cooperating fully, and everybody is trying to find a way to preserve it. It's just a sad thing in how it worked out this way."