Around South Florida's vast sugar cane fields, where turtles grow to the size of basketballs and alligators own the marsh, the silence of the swamp is broken by the sound of rumbling trucks and explosions.

The earth-moving equipment and high explosives are laying the foundation for a mammoth construction project: a reservoir bigger than Manhattan designed to revive the ecosystem of the once-famed River of Grass.

More than a century after the first homes and farms took shape in the Everglades, decades of flood-control projects have left the region parched and near ecological collapse. Now crews are building what will be the world's largest aboveground manmade reservoir to restore some natural water flow to the wetlands.

Engineers "built this thing beautifully," said Terrence Salt of the U.S. Interior Department, referring to the flood-control systems that practically drained the swamp to make way for development decades ago. "But as we look back at it through the lens of our current 21st-century values and understanding, you get a different take on it, which leads to our restoration efforts now."

The wetlands once covered more than 6,250 square miles, but they have shrunk by half, replaced with homes and farms and a 2,000-mile grid of drainage canals. In the process, the Everglades has lost 90 percent of its wading birds. Other creatures are at risk, too, including 68 species that are considered threatened or endangered.

The reservoir, estimated to cost up to $800 million, is the largest and most expensive part of a sweeping state and federal restoration effort.

Most man-made reservoirs are built in canyons or valleys and use a natural water source such as a river to fill in behind a dam. This one will stand on its own, contained within earth-and-concrete walls much like an aboveground swimming pool larger than many cities. Planners hope to eventually double its size.

Thomas Van Lent, a senior scientist with the Everglades Foundation, said the reservoir "is absolutely essential" to restoration efforts. But he acknowledges it will never return the region to its historical grandeur.

"There are parts you can restore completely, but you can't restore it all," he said. "It's probably unrealistic to expect Miami to move."

The Army Corps of Engineers, which is working with the state on restoration, recognizes the same limits.

"We're certainly never going to return it to the way it was 150 years ago," said the Corps' Stuart Appelbaum. "But we can do our best."

Water once flowed practically unhindered from the Everglades headwaters south of Orlando all the way into Florida Bay at the state's southern tip. But now when a hard rain falls, canals direct the overflow into the ocean to keep from inundating 5 million people who have settled in the area.

That's where the massive reservoir just south of Lake Okeechobee comes in. It will store up to 62 billion gallons of water that would normally be channeled out to sea and instead divert it into the Everglades at various times to mimic a more natural flow.

"We've developed about half of the Everglades, so we've got this very efficiently designed flood-protection system," Appelbaum said. Now engineers want to store that water so they "can put it back into the natural system to replicate what we lost when we did all the drainage."

Bulldozers and dump trucks are removing 30 million tons of dirt and muck from the reservoir site, which will then be surrounded by a 26-foot high, 21-mile levee of crushed rock and compacted soil. The levee will also have a 2-foot-thick concrete wall built into it to reduce seepage and add stability.

Major construction began in 2007. When the reservoir is compete in 2010, the shorelines will be so far apart _ 6 miles at the widest _ an onlooker won't be able to see from one side to the other.

The lake will be filled to an average depth of about 12.5 feet by diverting a nearby canal and adding pumps to push water into it. Officials also are considering allowing boating and fishing. The reservoir is almost sure to have alligators, too, since they are common throughout the Everglades.

No one disagrees that storing runoff water is key to reviving the Everglades, but the restoration effort has for years pitted environmentalists against the government.

The Natural Resources Defense Council has sued over the reservoir, claiming the state has not legally committed itself to using the water primarily for Everglades restoration.

The state insists 80 percent of the water will be for environmental purposes, but critics fear that without a legally binding agreement, the water could be sent elsewhere for agriculture or development.

"The Everglades and everyone deserves better than that," said council attorney Brad Sewell.

Other bodies of water planned throughout the Glades will serve in a similar way, but none will be as large as the 25-square-mile reservoir now being built.

The overall Everglades project, including the reservoir, is the largest such wetlands-restoration effort in the world. Much of its cost was supposed to be split 50-50 by the federal government and the state. But because Congress hasn't allocated its share, many aspects of the work have been delayed.

In 2000, the key parts of the restoration were estimated to cost $7.8 billion and take 30 years to finish. The price tag has now ballooned by billions of dollars because of rising construction and real estate costs. It's unknown when all the work will be complete, if ever.

While the restoration efforts have been slow-going, there are signs of success.

In the north, dozens of wading birds have returned to the Kissimmee River basin, the Everglades headwaters. In the south, a pair of newborn panther cubs were discovered last year near the Big Cypress National Preserve.

The big cats once roamed by the thousands throughout the southeastern U.S., but development has crowded out their only remaining habitat in southwest Florida. Scientists estimate there are no more than 100 panthers remaining in the state.

Carol Wehle, director of the South Florida Water Management District, said the birth of the panthers "can be directly attributed to restoration efforts."

"As we do these things, we're seeing how quickly Mother Nature actually heals herself," Wehle said.