A small army of researchers and university students lugging pick axes and shovels scattered across the country for three years to scoop samples into plastic bags from nearly 5,000 places. They marked the GPS coordinates, took photos and labeled each bag before mailing them back to the government's laboratory in Denver.
Though always underfoot and often overlooked, dirt actually has a lot to tell. Scientists say information gleaned from it could help farmers grow better vegetables and build a better understanding of climate change. A researcher of forensic science said mud caked on a murder suspect's boots could reveal if he had traipsed through a crime scene or had been at home innocently gardening.
David Smith, who launched the U.S. Geological Survey project in 2001, said data about the dirt will feed research for a century, and he's sharing it with anyone who wants it. "The more eyes and brains that look at it, the better," Smith said.
The idea for the massive research project came in the late 1990s, when Smith was in charge of handing out the government's store of soil data — what little there was.
The archive held information collected in the 1960s and 1970s. It was spotty and based on outdated science. Just about every researcher returned with the same disappointment, saying: "There must be more."
Smith told them that, sadly, no, there wasn't.
So he took action. During the next several years, Smith and his fellow geologists refined a plan for collecting and documenting the makeup of the nation's soil.
Digging started in 2007 and wasn't done until 2010. They strategically sunk their shovels at a spot in every 600 square miles. At each locale they took three samples — starting at the surface and going no deeper than three feet.
Before retiring, U.S. Geological Survey geologist Jim Kilburn trained many of the 40 surveyors and went into the field himself several times for up to a month. He sent back hundreds of samples on the road from Nebraska down to Texas and from Kansas west to the California coast.
Only once was Kilburn told to go away. A rancher near Sacramento, Calif., had let government researchers onto his pastures before, where they found a rare clover and told him he could no longer graze cattle there.
"No matter what I told the guy, he wasn't going to let me on," Kilburn said. "He had good reason."
A student Kilburn supervised caused a panic by leaving behind a sticky note on her motel room mirror with the reminder, "Send anthrax." The element occurs naturally in soil throughout the country, but it also has sinister uses. A housekeeper thought the worst, sparking a series of calls with geological survey headquarters until the confusion was resolved.
The hard work paid off. In October, the geological survey published a snapshot of minerals and chemicals in the ground. No other work captures the same information on a national scale, said Smith, who estimated the project cost $10 million.
Researchers at universities, institutes and government agencies have just begun using the data.
Kang Xia, a professor of environmental chemistry at Virginia Tech, stumbled upon the soil survey by chance — and at exactly the right moment.
She had set out to study and map the levels of organic carbon and nitrogen in soil — both critical for growing healthy crops. But she couldn't find samples of dirt from across the country.
"I was scratching my head," she said. "What do I do about this?"
Not long after that, a graduate student mentioned his summer job on the geological survey crew collecting dirt samples.
Problem solved. Xia emailed Smith, who offered her thousands of soil samples and a decade's worth of research.
Jennifer Phelan at RTI Inc., a research institute in Raleigh, N.C., is using the dirt to study acid rain's harm to forests, starting in Pennsylvania.
Cornell University professor Johannes Lehmann is leading a group of graduate students sifting through soil data for evidence of black carbon, a byproduct of forest fires and industrial smokestacks. The research may increase understanding of global warming.
"Without this sampling effort, we couldn't do this type of research," Lehmann said.
Dirt under the fingernails of a murder victim could help detectives figure out if the body was dumped there but killed elsewhere, said Sarah Jantzi, whose 2013 doctoral dissertation at Florida International University in the field of forensic science drew upon the government work.
A government scientist for 38 years, Smith said those after him can use this soil analysis as a starting point for decades of research. His data is available to anyone who thinks it will advance his or her project.
"The opportunities for further research are almost limitless," he said.
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