The IceCube observatory, built inside roughly one cubic kilometre of ice at the South Pole, detects only a few dozen of the most energetic cosmic neutrinos each year, and where they come from remains poorly understood.
The detector is the ice. Engineers drilled deep holes and lowered strings of photomultiplier tubes into them, using Antarctic ice that is unusually clear, with few bubbles or contaminants to scatter light.
What the tubes see are faint blue flashes of Cherenkov light, produced when a neutrino interacts in the ice and creates a fast-moving charged particle. From the pattern of that light, and for muon events from the shape of the cone it makes, the energy and arrival direction of each particle can be estimated.
The energies involved are difficult to hold in the head. Some of the ultra-high-energy neutrinos IceCube has caught carry about ten billion times the energy of a typical solar neutrino, and one detected by a similar experiment elsewhere had roughly 200 times more energy still.
The project's origins go back to the 1990s, when Francis Halzen recognised what the South Pole could be used for. This year he received the Nobel Prize in Physics for his contributions to IceCube and for the discovery of high-energy neutrinos of astrophysical origin.
Among the early papers was one written while the detector was still only planned, by Xiangdong Shi, George M. Fuller and Halzen, published in Physical Review Letters in 1998.
Fuller, now a Distinguished Professor of Physics at the University of California San Diego, is direct about what the highest-energy detections have told us. "We don't really know where that neutrino came from," he said.
He regards that as the point rather than a disappointment. "The fact that it got here at all may be saying something profound about the universe," he said. The account was published by UC San Diego and reported by ScienceDaily.
The origins of these particles, and the mechanisms capable of accelerating anything to such energies, remain among the most interesting open questions the instrument has produced.
It is worth stating plainly what has been achieved, because it is unusual. An instrument was built to detect particles that nothing else could see. It detects them. The result of detecting them is that we cannot say what they are from.
Every intermediate stage works. The ice is clear, the tubes are sensitive, the light is reconstructed, the energy is estimated and the arrival direction is recovered to within a few degrees. The direction then points at nothing anyone can name.
The finding, in other words, is an absence of explanation, delivered with coordinates. Our analysts regard this as the most precisely located ignorance in modern physics, and note that its precision improves every year.
The ice is a kilometre deep and almost perfectly clear. The view through it is excellent. At the far end of the view there is still nothing that anyone can put a name to.