LEAD, South Dakota — The gold rush glow has long faded from South Dakota, but a different kind of precious material is drawing crowds to the Black Hills. An old mine that produced billions of dollars in gold may be North America's best shot at finding dark matter. Until it closed in 2002, the Homestake Mine, nestled in the town that inspired the HBO drama Deadwood, was the oldest, largest and deepest mine in the western hemisphere. That tremendous depth makes Homestake the perfect hunting ground for rare, elusive particles that stubbornly refuse to interact with the rest of the world, like neutrinos and hypothetical particles that could explain dark matter. Similar detectors already exist in Italy, Japan, Canada and Minnesota. But the 8,000-foot-deep pit possible at Homestake would be deeper than them all, nearly as deep as Mount St. Helens is high. According to the lab's website, a deep lab at Homestake would more than double the world’s inventory of underground lab space. The National Science Foundation selected the Sanford Underground Laboratory at Homestake as the site of the new Deep Underground Science and Engineering Lab (DUSEL) in 2007, and physicists have already started moving in. Wired.com visited the mine-turned-lab to see the first glimmers of the dark matter rush. Image: Lisa Grossman/Wired.com

Deep Physics When the Homestake mine stopped hauling up ore in 2002, the old west town it left behind was at a loss. The pit that had kept the town afloat filled 4,500 feet deep with water. Jobs that had been stable for half a century disappeared. Even the high school football team — one of the best in the state when filled with husky miners' kids — started to suck, according to a high school senior who worked at the Black Hills Mining Museum over the summer. But the scientists flocking to that yawning hole in the ground are as eager as any gold profiteers. Their goal is summed up in a T-shirt sold in the lab's main office: "Nerds Searching for WIMPs." WIMPs are Weakly Interacting Massive Particles, one of the most popular candidates for the extra invisible stuff in the universe known as dark matter. About 83 percent of the matter in the universe is dark matter, which only makes itself known through its gravitational tugging on ordinary matter. Dark matter keeps galaxies from flying apart as they spin and is responsible for much of the large-scale structure of the universe, but is invisible to every telescope ever built. WIMPs, which are heavy, hypothetical particles that happen to be shy of regular matter, could occasionally collide with the nucleus of an ordinary atom. Anything that can hope to detect this rare, faint event needs to be hyper-sensitive, but that makes it vulnerable to false positives from cosmic rays and the particles left over when regular matter decays. Putting the detector deep in the Earth shields it from noise from space. The deeper, the better. Images: 1) The Yates Hoist Room, where miners once rode lifts up from 5,000 feet deep in the Earth in 3 and a half minutes. Now, the same trip takes 20 minutes. Credit: Lisa Grossman/Wired.com

The Homestake Experiment Homestake has a history as a place of physics. In 1965, astrophysicist Raymond Davis put a detector 4,850 feet deep in the mine designed to catch neutrinos, elementary particles that have almost no mass and can pass through ordinary matter undisturbed. His work ultimately confirmed that neutrinos do in fact have mass. Davis won the Nobel Prize in physics for this work in 2002, the same year the mine closed. Image: Sanford Underground Lab/Anna Davis

Searching for WIMPs The very same cavern that housed Davis' detector — now aptly called the Davis Cavern — will soon be home to the LUX (Large Underground Xenon) experiment: a 770-pound tank of liquid xenon waiting to catch a WIMP. Here's how it works: If a WIMP hits a xenon nucleus, it gives the xenon some extra energy, which makes that xenon move differently than the other xenon nuclei. That oddball xenon nucleus produces a faint flash of light, which is detected by photon-collecting tubes at the top of the tank. As the excited xenon nucleus moves through the tank, it strips a string of electrons off the other xenon nuclei. These electrons get caught in an electric field, and drift up into the thin layer of xenon gas at the top of the chamber to give off a brighter flash of light. By examining how bright these flashes are, physicists can reconstruct where the flash happened. They can also figure out the energy and mass of the incoming particle that sparked the flash in the first place, which is hopefully a WIMP. WIMP-hunting physicists take a cue from fictional detective Sherlock Holmes, who once said, "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." "If you see anything, you have to explain where it came from," said Jeremy Mock, a graduate student at the University of California, Davis who is part of the LUX team. "Once you've exhaustively explained where it came from, if it's still unexplained, it could be the signal you're looking for." Images: 1) The clean room where physicists are preparing LUX to go underground. 2) This copper structure will hold an array of photomultiplier tubes, which will detect the first flash of a WIMP hit. Credit: Lisa Grossman/Wired.com

Xenon Rush LUX uses so much xenon that the physicists who work on it joke that they've single-handedly driven the price of xenon up by thousands of dollars. The fact that other experiments still haven't seen any WIMPs puts upper limits on how much WIMPs have to weigh. Once it gets going, the physicists claim, it will take LUX less than two days to detect beyond those limits and zoom in on WIMPs' actual mass. Right now, LUX is getting ready in a converted Homestake warehouse. It should be ready to go 4,850 feet underground by December 2011. A lot of the scientists working at the surface lab are students devoting their summers to preparing the tank to go into the Earth. "If I can be here, why wouldn't I be?" Mock said. "This is where stuff is happening." Image: Lisa Grossman/Wired.com

Museum in the Making Preparing Homestake for a new life as a physics lab has inadvertently brought the mine's history out of the dark caverns and into the spotlight. When the NSF announced it was choosing Homestake for DUSEL back in 2007, they gave Carolyn Webber, assistant director of the Black Hills' historical Adams Museum, one year to get all the archives out of the mine. Those archives included cabinets full of survey maps, engineering drawings, frontier law records, geology records and more than 15,000 photos spanning the years from 1878 to 2001. Webber and her colleagues saw the exodus from the mine as an opportunity to build a museum, preserving and presenting the mine's colorful history. "It's the 126-year-old history of one of the longest, deepest, longest operating mines in the western hemisphere," she said. "People want to see them. Our whole goal is access." Images: Lisa Grossman/Wired.com

New Building, Old Books The museum-in-process, which will be called the Homestake Adams Research and Cultural Center, feels like a cross between a library and grandma's basement, full of boxes marked "Unidentified Archives" and curling, black-and-white photographs. But when the space is completed, it promises to be beautiful. Images: Credit: Lisa Grossman/Wired.com