Since the 1960s, every detector on Earth had counted roughly a third to a half fewer neutrinos arriving from the sun than the Standard Solar Model predicted. The discrepancy became known as the solar neutrino problem, and it persisted for decades. Either the models of how the sun generated energy were fundamentally wrong, or something was happening to the neutrinos during their eight-minute journey to Earth. One hypothesis held that neutrinos oscillated between three flavors -- electron, muon, and tau -- and that earlier detectors, sensitive only to electron neutrinos, were missing the ones that had changed form along the way. Proving this required a detector that could count all three flavors simultaneously.

In 1984, physicist Herb Chen of the University of California at Irvine proposed a detector using heavy water, in which hydrogen atoms carry an extra neutron. Heavy water enabled two distinct detection reactions: one sensitive only to electron neutrinos, the other sensitive to all three flavors. Canada was the ideal location because Atomic Energy of Canada Limited maintained enormous stockpiles of heavy water for its CANDU reactor power plants and was willing to lend the needed supply at no cost. The Creighton Mine in Sudbury, among the deepest in the world, offered minimal background radiation. The SNO collaboration held its first meeting in 1984, won federal funding over the competing TRIUMF KAON Factory proposal, and received the official go-ahead in 1990. Mining of the detector cavity -- the largest in the world at such a depth -- was completed in May 1993.

The finished detector was an engineering marvel. A 12-meter acrylic vessel held the heavy water, surrounded by a cavity filled with ordinary water for buoyancy and radiation shielding. Approximately 9,600 photomultiplier tubes, mounted on a geodesic sphere, watched for the telltale blue glow of Cherenkov radiation produced when a neutrino interaction created a relativistic electron. The entire facility was maintained as a clean room -- most areas at Class 3000, the detector cavity itself at the stricter Class 100 standard. The observatory sat at the end of a dedicated drift isolated from other mining operations, a precision physics laboratory hidden inside an active nickel mine.

On June 18, 2001, SNO published its first results: clear evidence that solar neutrinos oscillate, transforming from one flavor to another during their journey from the sun. The total neutrino flux across all three flavors matched the Standard Solar Model's predictions. The sun was producing exactly as many neutrinos as theory predicted; earlier detectors had simply been blind to the ones that had changed form. The discovery implied that neutrinos possess mass, a finding with profound implications for the Standard Model of particle physics. Japan's Super-Kamiokande had published evidence for neutrino oscillation in 1998, but those results involved atmospheric neutrinos and were not conclusive for solar neutrinos. SNO provided the definitive solar proof.

In 2015, the Nobel Prize in Physics was awarded jointly to SNO's director, Arthur B. McDonald, and Takaaki Kajita of Super-Kamiokande for the discovery of neutrino oscillation. The SNO team had already received the inaugural John C. Polanyi Award in 2006 and the 2016 Fundamental Physics Prize. An asteroid, 14724 SNO, was named in the experiment's honor. The underground laboratory itself has been enlarged into SNOLAB, a permanent facility hosting multiple experiments, while the original SNO equipment was refurbished for use in the successor SNO+ experiment. The mine that once produced only nickel now produces knowledge about the fundamental nature of matter.