Super-K works by exploiting a phenomenon called Cherenkov radiation -- the optical equivalent of a sonic boom. When a neutrino, arriving from the Sun or the atmosphere or a distant supernova, interacts with a water molecule inside the tank, it can produce a charged particle that moves faster than the speed of light in water. That particle generates a cone of blue light that fans outward and strikes the walls as a ring. The 11,146 photomultiplier tubes lining the inner detector record the precise timing and intensity of each ring, allowing physicists to determine the incoming neutrino's energy, direction, and type. Sharp-edged rings indicate muons; fuzzy rings mean electrons. From these faint halos of Cherenkov light, painted on the walls of an underground tank, some of the deepest truths about the universe have emerged.

The story begins in 1982, when the University of Tokyo's Institute for Cosmic Ray Research started construction on a detector called KamiokaNDE -- Kamioka Nucleon Decay Experiment -- inside the Mozumi Mine. Its original purpose was to catch protons in the act of decaying, a prediction of Grand Unified Theories that would confirm the deep unity of nature's forces. Proton decay was never observed, but the detector was upgraded in 1985 to detect solar neutrinos, and in February 1987 it captured ten neutrinos from Supernova 1987A in the Large Magellanic Cloud -- the first time neutrinos from a stellar explosion had ever been recorded. That success led directly to the design of a far larger successor. Approved in 1991 for approximately $100 million, Super-Kamiokande held fifteen times the water volume and ten times the photomultiplier tubes of its predecessor. It began taking data in April 1996.

In 1998, Super-Kamiokande delivered one of the most important results in the history of particle physics. By counting atmospheric neutrinos -- secondary cosmic rays produced when protons from deep space slam into the upper atmosphere -- the team discovered that muon neutrinos traveling upward through the Earth arrived in far fewer numbers than those coming downward from directly overhead. The only explanation was that the upward-traveling neutrinos, having journeyed through thousands of kilometers of rock, had oscillated into a different flavor that the detector could not see. Neutrino oscillation requires neutrinos to have mass, something the Standard Model of particle physics had assumed was impossible. The finding forced a revision of fundamental physics. In 2015, Super-K researcher Takaaki Kajita was awarded the Nobel Prize in Physics alongside Arthur McDonald of the Sudbury Neutrino Observatory for confirming this result.

On November 12, 2001, disaster struck. A single photomultiplier tube, probably near the bottom of the water-filled tank, imploded. The shock wave from the implosion cracked a neighboring tube, which imploded in turn, and within seconds a chain reaction had destroyed approximately 6,600 of the detector's 11,146 photomultiplier tubes. The physics was grimly elegant: each imploding vacuum tube generated a pressure wave powerful enough to shatter its neighbors, cascading through the array like falling dominoes. The detector was partially restored by redistributing the surviving tubes and encasing each one in a protective acrylic shell to prevent any future chain reaction. By 2006, after the installation of roughly 6,000 replacement tubes, Super-Kamiokande was fully rebuilt. It has operated continuously since, through several upgrade phases, most recently the addition of gadolinium sulfate to the water in 2020 -- a modification that allows the detector to distinguish neutrinos from antineutrinos for the first time.

Super-Kamiokande is not only an instrument for studying known physics. It is a sentinel, watching for rare events that may never come. It continues to search for proton decay, setting a lower bound on the proton's half-life at 5.9 times 10 to the 33rd years for certain decay channels. It stands ready to detect a burst of neutrinos from a supernova in the Milky Way, an event expected to produce 100 to 150 detectable interactions -- enough to reconstruct the direction of the explosion in real time. And through the T2K experiment, which fires a beam of neutrinos 295 kilometers from Tokai to Kamioka, it probes the subtle differences between matter and antimatter that may explain why the universe exists at all. From above, there is nothing to see -- just forested mountains and the quiet valley of Hida. But a kilometer below, in perfect darkness, 50,000 tonnes of water wait for the next flash of blue light.