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Every December since 2004, engineers have flown to the South Pole to drill 8,000-foot-deep holes in the ice. The team lowers cables, each strung with 60 disco-ball-size light sensors, into the holes and let them freeze over. So far they have completed 79 such holes, set in a grid half a mile on each side, and plan to drill the final seven this month. The result will be the IceCube Neutrino Observatory, a cube of ice packed with 5,320 sensors looking for cosmic particles.
Neutrinos are subatomic particles created by radioactive decay or nuclear reactions. Like other types of extrasolar radiation, they emerge from energetic cosmic events and constantly bombard Earth. Neutrinos are unique among cosmic particles, however, in that they carry no electric charge. The magnetic fields of stars and planets bend the paths of charged particles, making it impossible for scientists to identify their origin. But neutrinos fly in a straight line: Catch one, and you can trace it back to whatever produced it, which makes them one of the easiest means of probing the far reaches of the universe.
Detecting a neutrino, however, is a bit like trying to catch a flea with a fishing net—the particles are so small that trillions of them travel through Earth every second without even hitting an atom. So the researchers at IceCube employ a clever technique to spot indirect evidence of neutrinos
Every day, several dozen neutrinos passing through IceCube will hit a hydrogen or oxygen atom in the ice and eject another particle, called a muon, that emits a blue light. In Antarctica’s nearly pure ice, the photo sensors can spot such a flash a football field away, and with dozens of sensors registering each muon, scientists can triangulate the neutrino’s exact path through the ice and extrapolate it to its source.