When the City of Industry bought the 2,450-acre Tres Hermanos Ranch, preservationists cried foul. They became unhinged after hearing the business-centric city planned a series of reservoirs and hydro-power plants that would destroy the oak-and-walnut woodland.

But after nearly four decades, nothing ever happened. Now, in a twist of fate, development of the greenbelt may become reality.

Thirty-seven years later, the nearly untouched land in Chino Hills and Diamond Bar was pried from the hands of the Industry Urban-Development Agency by the state as a condition of ending redevelopment. Last month, the former redevelopment agency turned over the pristine swatch of land to a commercial real estate broker. Now, one of the biggest bidding wars in the history of Southern California real estate is only days away.

The pending sales touched off an angry response from residents who don’t want to see hillsides turned into strip malls and condo complexes. It also provides a new opportunity for preservation groups to buy the land. Like a toppled dictator, the City of Industry’s grip on Tres Hermanos has slipped, creating a free-for-all atmosphere and the biggest development threat on land loaded with trees, bobcats and raptors since city ownership began in 1978.

“This is a spectacular piece of property,” said broker Graham Gilles from The Hoffman Co., an Irvine-based firm handed the rights to sell the land to the highest bidder starting the end of the month. “It is 2,500 acres that I don’t think exists in California today.”

The land is split: 1,700 acres are in Chino Hills and 720 acres in Diamond Bar. City of Industry officials said it could fetch $125 million for the state. Well-heeled developers are lining up to file a bid, Gilles said, with speculation the former cattle-grazing ranch could become hundreds of homes, a commercial center or both. But the possibility of homes and strip malls carved into one of the last untouched spaces in the region has spurred a nascent effort from Diamond Bar and Chino Hills residents who want to see the land preserved.

“It sounds simplistic. But even if it is just a minute, being able to look into the distance and not see any human structures. It has an effect on you. It is almost like a relief from all the urban everything we are constantly surrounded by,” explained Dan Swenson, 45, of Chino Hills, who has started a letter-writing campaign to keep the land undeveloped.

A “Save The Tres Hermanos Ranch” group emerged on Facebook about six weeks ago after news of the pending sale went public. The group met at the Diamond Bar Community Center on May 31 and has grown to about 400 members.

“That thought of taking our last open lands, with cows and pastures and a little lake, really bothered people. So, about three or four of us got together to see if this could be mitigated,” said Brian Worthington, 48, a spokesman for the group who is also running for Diamond Bar City Council in November.

The group is suggesting numerous options, from a regional park, to part development/part parkland to outright purchase by way of a Kickstarter online campaign to raise cash.

They’ve attracted students from Cal Poly Pomona’s John T. Lyle Center for Regenerative Studies, who are working on a preservation plan, wrote Ashley Cupp, a regenerative community fellow with the center, on the group’s Facebook page.

Swenson has written a letter to Kelly Elliott, Inland Empire District superintendent for the California State Park, suggesting the state annex the land into the existing 4,000-plus acre Chino Hills State Park to the south.

Elliott, in a June 4 letter, responded by saying the state parks department is holding off on new land purchases because it is reorganizing its acquisition department. However, Elliott did not completely close the door.

“The Tres Hermanos property is one of many areas identified as desirable additions to Chino Hills State Park with current willing sellers,” she wrote, adding that the state has met with the Trust for Public Land regarding buying a parcel of land near Prado Dam.

“Inland Empire District staff continues to explore options and collect information during this interim period, regarding potential acquisitions,” she wrote.

Swenson pitched the idea of cities banding together to buy open space. Monrovia residents voted to tax themselves to buy land in the San Gabriel Mountain foothills in 2000. Swenson mentioned Boulder, Colo.’s open space management efforts in a letter to Joann Lombardo, director of community development for Chino Hills.

“She responded by saying open space preservation would be dealt with through the development process,” Swenson said.

Swenson, who has a doctoral degree in environmental science and engineering from UCLA, says he’s cautiously optimistic. He doesn’t want to see the land sold to a developer and the possibility of some amount of preservation a defacto response to more density and traffic, especially on already-clogged Grand Avenue and the 60 and 57 freeways.

“That would be unfortunate if we would lose that opportunity,” he said.