Thousands of meters below the ice near the South Pole lies one of the most unusual observatories ever constructed. The instrument's nervous system comprises 86 strands of light detectors, stretching down into the ice sheet like oversize strings of pearls. Each strand features 60 basketball-size detectors, spanning the depths from 1,450 to 2,450 meters below the surface. And the body of the observatory is the ice itself, an abundant medium with an astonishing natural clarity.

Altogether, the instrument, known as IceCube, spans a cubic kilometer of ice. Scientists have for years been taking data using the partially built observatory, but on December 18 the 86th and final string of detectors was lowered into place, marking the completion of construction on the estimated $270-million project. The observatory will likely start running at full strength in April, according to communications manager Laurel Bacqué.

IceCube's task is to watch for energetic neutrinos emanating from violent cosmic events such as supernovae and gamma-ray bursts. Neutrinos are a curious brand of particle—electrically neutral and loath to interact much with other particles, they zoom from their cataclysmic origins through the intergalactic medium and can pass through Earth unscathed. (They also originate from more mundane sources such as the sun.) Much as gamma-ray and x-ray observatories have already done, IceCube should provide a new layer of observational insight to the highest-energy processes in the universe. For instance, catching the trace of neutrinos from core-collapse supernovae, some of which emit more than 99 percent of their energy in neutrino form, would allow astrophysicists a clearer look into the mechanism by which stars die.

IceCube takes advantage of neutrinos' slipperiness, using Earth as a filter of more interactive particles. From the South Pole the observatory hunts out particles that come barreling out of the northern sky; some strike atoms at Earth, some pass straight through the planet, and a critical few pass nearly all the way through, striking an atom in the last few kilometers of Antarctic ice instead.

When a neutrino does strike an atom in IceCube's cubic-kilometer expanse, it gives off a high-speed burst of charged secondary particles, such as muons, that illuminate the transparent ice with a brief flash of light. IceCube's array of detectors, more than 5,000 in total, can then determine the origin of the neutrino based on the trajectory of the secondary particles. (Northern-sky neutrinos come streaking upward through the detector, from the bedrock toward the surface of the ice.) The beauty of neutrino astronomy is that, being neutral, the particles trace a straight line back to their point of origin.

IceCube can also detect more local neutrinos and muons that originate from cosmic rays striking Earth's atmosphere. Those particles essentially constitute the background that scientists will have to filter out to find distant astrophysical neutrino sources, but they are providing some surprises in their own right. "We already made the totally puzzling observation that an excess of galactic cosmic rays reaches Earth from a spot pointing at Vela, the strongest gamma-ray emitter in the sky," says University of Wisconsin–Madison physicist Francis Halzen, the project's principal investigator. Because cosmic rays are charged, their inbound trajectory should be jumbled by the Milky Way's magnetic field, so any hot spot on the cosmic ray map demands an explanation.

With a full-power observatory in place, that explanation, as well as a better explanation of high-energy astrophysical phenomena throughout the universe, may be available in the years to come. "A great asset of IceCube so far is that we took data as the detector increased in size," Halzen said as the final hole was being drilled. "It will, however, be great to finally have a stable instrument that we can calibrate and fine-tune without any further major changes. So, as soon as the celebration stops, we will start preparing for a long, stable, uninterrupted period of data taking."