In one of the more unusual outdoor endeavors during this streak of frigid weather, a dozen archaeologists are excavating a cluster of ancient campsites in Chanhassen.

They're doing so in a race to save the past from the future: When the weather warms up in a few months, the sites will be destroyed by road construction.

The area contains bones, stone tools and other evidence of the people who lived there 3,000 to 8,000 years ago.

But there's a catch: The artifacts are 10 feet underground, beneath a wetland. That has raised huge challenges — especially in the heart of winter — to excavating, screening and recording the findings.

The discovery is one of the most important in Minnesota history, said state archaeologist Scott Anfinson. "These sites are rare," he said. "Half of Minnesota's prehistory is during this period, and we hardly know anything about it."

Archaeologist Frank Florin found artifacts in the area after Carver County hired his company, Florin Cultural Resource Services, to sample the area. Test digs in 2012 and last year were required by federal rules to see if anything of historic value will be disturbed in certain road projects.

This spring, the area will be greatly disturbed when workers install deep footings as part of a $54 million replacement of the Hwy. 101 bridge linking Carver County and Shakopee. So Florin and a team have switched from sampling to excavating in seven locations to rescue what they can before work begins on the bridge and its approach along County Road 61, also known as Flying Cloud Drive.

The cost of the testing and excavations will be about $290,000, said Lyndon Robjent, Carver County's public works director.

Seeking treasures

At one of the sites last week, workers in heavy clothes and boots clumped up and down pallets sloping into a swimming-pool-sized excavation. The bottom consisted of runny, jet-black mud covered by more pallets and plywood sheets to allow access and prevent freezing.

One crew used flat-ended shovels to scrape layers of mud off the surface and slop it into buckets. They worked on one square meter at a time, removing 10 centimeters of mud per scoop.

Others hauled the buckets by hand to a couple of plastic tents with portable heaters, where archaeologists placed the dough-like mud into screens, lowered them into stock tanks filled with water, and proceeded to squish the gooey black through the screens to find artifacts. Others were ready to bag and record whatever was found.

"It looks like a big treasure hunt, but it's really as careful a recovery as we can make happen," said Bob Thompson, an independent archaeologist from Maple Grove, working one of the shovels.

Despite the cold and mud, Thompson was enthusiastic about being at the site by 5 a.m. to start running the pump and removing water so the crew could start work at 8. Others removed Styrofoam sheets placed on the mud overnight to keep it from freezing. "If you're an archaeologist, you have to get up from your microscope once in a while and get into the field," Thompson said.

Unexpected benefit

Winter archaeology in Minnesota is rare, Anfinson said. Only five places — usually small sites — have been excavated during the winter in the past 30 years. In Chanhassen, he said, working in winter may be an advantage because crews need to dig through wetlands.

"What happens when you dig in the mud is that the walls start slumping because they're saturated with water," he said.

But in this case, frigid weather froze the walls and reduced the flow of water seeping up from below, Anfinson said. "These aren't good conditions to do archaeology, but considering the environment there, it actually turned out to be a benefit," he said.

Florin said the site is about a half-mile long and contains evidence of multiple campsites that were on the shore of a large lake near the Minnesota River from 3,000 to 8,000 years ago.

Paradise then

Those who used the area were small groups of people who probably camped, hunted and fished for a few days and then moved on, Florin said. The area was rich in wild rice, turtles, muskrats, beavers, deer and bison, he said, with clear springs for drinking water and sources of raw material for making stone tools.

"It probably would have been a paradise that provided everything that one could need," Florin said.

That is also why the area was used for so long, he said. Findings have included hundreds of items, including an 8,000-year-old fire hearth, a variety of bones from bison and other animals, pieces of chert (a type of quartz), flint and other debris from making tools, and stone scrapers, knives and spear points.

After the items are numbered, cataloged, photographed and analyzed, Florin said, they will be archived at the Minnesota Historical Society.

"One of the reasons why this site is so important is that it contains a well-preserved record of how people lived at that time," he said.

Anfinson said only a few sites of that age have been excavated in the state, mostly camps in western Minnesota used by bison hunters. "We don't know what people in the eastern half of Minnesota were doing at this time," he said. More sites have not been found because the shallow ones in upland areas were plowed and disturbed by European settlers, he said. And ancient campsites in the river valley, buried by centuries of erosion and sedimentation, have been too deep and too expensive to search.

Anfinson said the state archaeologist's office will receive about $150,000 later this year for new initiatives, and much of it may be used to explore for sites elsewhere in the Minnesota River Valley that are not facing imminent bridge or road projects.

"We're hoping to find deep sites in great condition that could be dug some day," he said.