On a gray midwinter dawn, a dozen scientists and technicians in hard hats gather outside the entrance to the Soudan Mine. Wearing heavy coats and clutching lunch bags, they step expectantly into the open cage of an 80-year-old hoist. An ancient engine chuffs as it lowers a steel gondola down the mine’s 2,341-foot-long shaft. The trip takes three minutes, but the passengers do not talk much. Instead, they listen idly to the icy air swooshing upward as the cage plunges into the guts of northeastern Minnesota’s Iron Range, away from the snowdrift-filled landscape and into a world of darkness.
The scene echoes an ordinary day at the mine more than a century ago. The Soudan Mine opened in 1884 atop one of the richest iron deposits in the world—hard hematite, about 65 percent pure. The ore quality kept Soudan going until 1962, when U.S. Steel decided that the mine had become too deep and hence too expensive to operate. New techniques made it cheaper to process lower-quality ore somewhere else. But the great depth that condemned the mine as a commercial venture has more recently given it new life as a research outpost, the Soudan Underground Laboratory. The half-mile layer of overlying rock creates a shield of peace and quiet unattainable anywhere on the surface of the earth. Physicists now burrow down deep here, oddly enough, to mine the heavens.
The gondola reaches level 27—the very bottom—and opens onto a state-of-the-art particle physics lab carved from two high-ceilinged caves. This is the home of the Cryogenic Dark Matter Search, inevitably abbreviated as CDMS.
The goal of CDMS is to hunt down another physics acronym: WIMPs, or weakly interacting massive particles. These enigmatic, hypothetical particles are the leading suspects in the search for dark matter, the unseen bits of whatever that are thought to make up the bulk of the matter in the universe. Ordinary atomic matter may seem like everything. After all, we are talking about all the stars as well as planets, comets, moons, the Crab nebula, black holes, brown dwarfs, the Pacific Ocean, you, me, cans of soup, and the family dog—all of it. Seven decades of astronomical research says that we are missing the big picture, however. There is four times as much dark matter as there is so-called ordinary matter, and we know hardly anything about it.
Today there is little doubt that dark matter exists. “It was very difficult at the beginning to imply that there were things out there that you couldn’t see,” says CDMS project coleader Bernard Sadoulet, an astrophysicist at the University of California at Berkeley. “Modern measurement techniques have authenticated the observations, but the problem is to detect whatever it is.” Whoever finds some is a virtual lock for the Nobel Prize. Actually uncovering a sample is . . . well . . . another matter. That is what draws researchers into the bowels of an abandoned Minnesota iron mine, working long shifts in what is surely one of the most tedious, yet potentially rewarding, jobs in all of physics.
