Early prospectors digging for gold, silver and copper across the American West had no idea that one day something else very valuable would be buried in the piles of dirt and rocks they tossed aside.
America is scrambling to find key components of mobile phones, televisions, weapons systems, wind turbines, MRI machines and regenerative brakes in hybrid cars - and old mine tailings piles might just be the answer.
One era's junk could turn out to be this era's treasure because experts say the tailings may contain a group of versatile minerals the periodic table called rare earth elements.
"Uncle Sam could be sitting on a gold mine," said Larry Meinert, director of the mineral resource programme for the US Geological Survey in Reston, Virginia.
The USGS and Department of Energy are on a nationwide hunt for deposits of the elements that make magnets lighter, bring balanced hues to fluorescent lighting and colour to the touchscreens of smartphones in order to break the Chinese stranglehold on such goods.
But they were surprised to find that the critical elements could be in plain sight in piles of rubble otherwise considered eyesores and toxic waste.
"Those were almost never analysed for anything other than what they were mining for," Mr Meinert said. "If they turn out to be valuable that is a win-win on several fronts - getting us off our dependence on China and having a resource we didn't know about."
The 15 rare earth elements were discovered long after the gold rush began to wane, but demand for them only took off over the past 10 years as electronics became smaller and more sophisticated.
They begin with number 57 Lanthanum and end with 71 Lutetium, a group of metallic chemical elements that are not rare as much as they are just difficult to mine because they occur in tiny amounts and are often stuck to each other.
Unlike metals higher up on the table such as silver and gold, there is no good agent for dissolving elements so closely linked in atomic structure without destroying the target. It makes mining for them tedious and expensive.
"The reason they haven't been explored for in the US was because as long as China was prepared to export enough rare earths to fill the demand, everything was fine - like with the oil cartels," said Ian Ridley, director of the USGS Central Mineral and Environmental Resources Science Centre in Colorado.
"When China began to use them as a political tool, people began to see the vulnerability to the US economy to having one source of rare earth elements."
Two years ago, China raised prices - in the case of Neodymium, used to make Japan's Toyota Prius electric motors stronger and lighter, from 15 dollars a kilogramme in 2009 to 500 dollars in 2011, while Dysprosium oxide used in lasers and halide lamps went from 114 dollars a kilogramme in 2010 to 2,830 dollars in 2011.
When China cut off supplies to Japan in a dispute over international fishing territory, the US government went into emergency mode and sent geologists to hunt for new domestic sources.
At the University of Nevada-Reno and Colorado School of Mines, USGS scientists used lasers to examine extensive samples of rocks and ore collected across the West during the gold rush days by geologists from Stanford University and Cal Tech.
"If we could recycle some of this waste and get something out of it that was waste years ago that isn't waste today, that certainly is a goal," said Alan Koenig, the USGS scientist in charge of the tailings project.
One sample collected in 1870 from an area near Sparks, Nevada, where miners had searched for a viable copper vein, has shown promise and has given researchers clues in the search for more. They have found that some rare earths exist with minerals they had not previously known occur together.
"The copper mine never went into production, but now after all of this time we've analysed it and it came back high with Indium, which is used in photovoltaic panels. It never economically produced copper, but it gives us insight into some associations we didn't previously recognise," Mr Koenig said.
Indium also has been found in the defunct copper mine that dominates the artsy southern Arizona town of Bisbee.
Mr Koenig and his colleagues are working to understand the composition of all of the nation's major deposits sampled over the past 150 years. In some cases, the mines were depleted of gold or copper, but the rocks left piled alongside mines and pits could hold a modern mother lode.
"We're revisiting history," he said.