The numbers are big: nearly 93,000 acres, 144 square miles and a half-million new Wasatch Front residents in the next 75 years.

And behind it all is a fairly young company, Kennecott Land, the little sister of mining company Kennecott Utah Copper. Formed in April 2001 by Rio Tinto, the international mining company that owns Kennecott's mining arm, Kennecott Land now has just over 50 employees.

Since the early 20th century, Kennecott has been mining in the Oquirrh Mountains, accumulating a mass of land not just for mining but also for tailings ponds, transportation, buffers and other uses. That land — now almost 93,000 acres, with about 79,000 of those acres lying in Salt Lake County — is the largest metropolitan landholding by a single owner in the United States.

And it's just about ready for the next phase.

As Kennecott's mining operations wind down and change focus, the land is no longer needed to support that use. So Kennecott Land was created to figure out just what to do with the property, Kennecott's "asset in addition to mining," Kennecott Land President Peter McMahon said.

There were options. Kennecott Land could have sold the acreage off in chunks to developers to build their own subdivisions, commercial developments, office parks, industrial zones and any other variety of uses. Or Kennecott Land could opt to plan and develop the whole thing itself. The company went with the latter option.

"It made more sense to keep it in the family, so to speak," McMahon said.

The result has often been called "unprecedented" — a countywide effort to master-plan the entire development, drawing off ideas discussed in four summits held since August. The summits have brought together county planners, officials from nearby cities, environmentalists, transportation planners, public utilities and a host of other interested parties, all to gather input and guide Kennecott Land's vision for the next 75 years.

"The land is large," McMahon said. "We're not just a developer with 500 acres. . . . If you're building at this scale, you can't do it without engaging the community."

And it's a big community that's involved.

"This Kennecott project doesn't just affect Kennecott and that 93,000 acres," county Mayor Peter Corroon said. "It affects the whole county. We're talking about another half-million people moving to the valley in the next 50, 60 years."

The land

Not since the early settlers arrived in the Salt Lake Valley has there been so large a clean slate for development, Kennecott Land officials are fond of saying. And rarely has there been anywhere in the United States such a large-scale plan for tackling so huge a piece of undeveloped property.

All of Kennecott's land lies within 20 miles of downtown Salt Lake City. It runs along the Oquirrh Mountain foothills and all the way to the top of the mountains, in some places crossing over into Tooele County.

Of the 79,000 acres lying in Salt Lake County, 4,000 acres have already been planned as South Jordan's Daybreak community, Kennecott Land's first project in which 800 homes have already been built, an 85-acre lake is being filled and a school and community center serve more than 1,100 children and their families. When Daybreak is finished, the community will feature 13,000 homes .

Another 34,000 acres will remain untouched as open space, much of that lying on steep slopes along the mountain range.

The rest of Kennecott's land project — about 41,000 acres — will be developed, piece by piece, following the county's adoption first of the general principles of the plan and then of the specific land uses. Kennecott Land expects it will take about 75 years before all the development is complete. What will happen after Daybreak has yet to be announced.

While the land has been used for more than a century for mining, McMahon said only about 13,000 or 14,000 acres have ever actually been part of the mining footprint. The rest, he said, has been "effectively untouched."

Still, in 1999 Rio Tinto conducted the country's largest voluntary remediation work to clean up any residue.

McMahon said the first developments will be on that untouched land and that it will be many decades before any development begins on land used for mining or tailings.

The plan

Kennecott Land's current master plan for the development centers on a mass transit "spine" running from Daybreak in the south to a large urban development to the north, off I-80 north of Magna. The line would include light-rail spurs connecting to existing and planned lines throughout the valley.

Along the transit spine would be the bulk of the development — from small neighborhoods and grocery store developments to large urban developments packed with multifamily housing and 1 million square feet of commercial space.

Farther west, small, low-density foothill developments would pop up here and there, with the possibility of a skiing area near Soldier Flats in the top of the Oquirrhs.

At this point, Kennecott Land has no stated preferences as to the jurisdiction of its future developments — whether they remain parts of unincorporated Salt Lake County, are annexed into existing cities or if new cities will be formed. County planning director Jeff Daugherty said the county hopes to adopt plans and principles that will be flexible and can follow the development no matter whose control it ends up under.

The plan has received preliminary support from the dozens of summit participants who watched the plan evolve and gave input in recent months. Its backing by so many interests comes from Kennecott Land's unique process and specific ideas, McMahon said.

"We've been able to paint a vision," he said. "We've sort of captured their imaginations a bit."

The planning has often centered around concepts promoted by Envision Utah, a public-private group pushing for "smartly planned" growth. Though Envision Utah is not a partner in Kennecott Land's plans, the organization is watching them closely.

"From our perspective there are still issues to be addressed, but we think it's admirable that a private landowner would go to extraordinary lengths to make sure that its project meets the long-term transportation, environmental needs of the region," Envision Utah executive director Alan Matheson said. "It's early in the process so we want to continue to communicate with them in finding solutions that will work for this area. We're talking still 70 years out, and a lot can happen in that time. But I think we're fortunate to have a company willing to put in the resources" to plan from the beginning.

But not everyone is eagerly awaiting the development.

Lying in the mouth of Bingham Canyon, near Kennecott Utah Copper's massive copper pit, is the township of Copperton, a quiet, unincorporated little piece of Salt Lake County that residents worry is on the verge of big changes, spurred largely by Kennecott Land's plans.

They worry that nearby cities will seek to annex Copperton, while most residents there want the area to remain unincorporated. And they worry about the commotion and chaos of suburban sprawl moving closer to their doorsteps.

"What's around us we really can't control, but we want to keep our community intact the way it is," Town Council chairman Jerry McCalmon said.

The environment

One of Kennecott Land's stated principles is environmental sustainability, and McMahon said Rio Tinto's commitment to environmentally friendly policies was a big reason the company decided to keep control of the development rather than selling it off.

Kennecott's plans call for a focus on walkability and mass transit, as well as sustainable energy use, water conservation and maintaining about 30 percent of the developed land as open space.

The company recently received ISO-14001 certification, an international environmental-standards certification typically given to large manufacturers. Kennecott Land is the only land developer in the country to be ISO certified, McMahon said, adding that Tinto has a policy that all of its subsidiaries will receive the certification.

"It says we're pretty much at the top of the class in how we manage environmental protection," he said.

Local environmentalists are cautiously optimistic about Kennecott Land's plans.

Marc Heileson, regional representative at the Sierra Club, said he sees promise in the plans, especially in the work of Calthorpe Associates, the company behind Kennecott's plan.

"One thing we are pleased with is their decision to go with Peter Calthorpe," Heileson said. "He has a long reputation of some very good smart-growth developments that have been built around access to mass transit and other things that make good growth. What we've seen so far of the layout plan does have a heavy transit focus."

Heileson said growth is inevitable, and the Sierra Club just wants to see that growth managed well.

"We're not against growth, but we do acknowledge that the patterns of development determine our air quality, our open space, our water quality and, more important, our quality of life," he said. "Are we stuck in bumper-to-bumper traffic under a brown cloud of pollution for the rest of our lives?"

He said for the plan to be successful, planning, funding and construction on east-west light-rail lines need to be a top priority for transportation planners. He said the Sierra Club will continue to watch Kennecott Land's plans and hopes they are carried out as they have been explained so far.

He would also like to see a focus on renewable energy sources, saying solar and wind power could be very successful on the west bench and that planning new communities is the ideal time to incorporate alternative types of energy.

The demand

But are such ambitious plans realistic? Will the Wasatch Front continue to see the rapid growth it has in recent decades, or are projections that there will be 500,000 new people looking to move to the Salt Lake Valley in the future overly optimistic?

Jaren Davis, the president of the Utah Association of Realtors, thinks the projections are conservative.

"The Wasatch Front spent years hidden from the world as to what we are, what we have to offer," he said. He said the area has a lot considering its small population base, from the arts to sports to shopping to recreation.

Recent moves by large national companies like IKEA and Cabela's to build in the area "bode well" for the idea that a lot of growth is still on its way, Davis said, and that jobs will follow.

"There's just a lot to our area, to our culture, to our people that encourage businesses to want to be here," he said, including the Salt Lake area's proximity to freeways, the airport, railways, an educated work force and lots of bilingual residents.

Researchers at the University of Utah have predicted 106,000 jobs will spring up along the west bench, with the entire project, including construction and other off-site jobs, generating as many as 262,000 new jobs in the county.

Davis said he is already liking what he has seen from Kennecott Land and its ability to set a framework for that growth.

"I think Daybreak, for example, is very good at master-planning, being able to go in and anticipate what's coming, and build into their model an opportunity for walkability, for there to be parks, schools, churches, affordable housing and a variety of housing," he said.

He said Utah's largest hurdle in the past has often been outsiders' skewed perspectives of the culture, but he sees that changing.

"The world will recognize Salt Lake as a good place to raise families and even non-traditional families," he said.