The story begins in 1983, when the original Kamiokande detector -- short for Kamioka Nucleon Decay Experiment -- started watching for protons to fall apart inside 3,000 tons of water deep in a working zinc mine. Proton decay never showed up, but in February 1987 something extraordinary did: a burst of neutrinos from Supernova 1987A in the Large Magellanic Cloud, 168,000 light-years away. Masatoshi Koshiba and his team detected 11 of those neutrinos, helping to found the field of neutrino astronomy and earning Koshiba a share of the 2002 Nobel Prize in Physics. Super-Kamiokande followed in 1996 with 50,000 tons of water and proved that neutrinos oscillate between three flavors, earning Takaaki Kajita a share of the 2015 Nobel Prize. Hyper-Kamiokande inherits this legacy and amplifies it, with 8.4 times the fiducial volume of Super-K.

The detector works on an elegantly simple principle. Ultra-pure water fills the enormous cylindrical tank -- 68 meters in diameter and 71 meters tall -- while thousands of photomultiplier tubes watch for the faintest blue glow. When a neutrino strikes a water molecule, the resulting charged particle travels faster than light moves through water, emitting a cone of Cherenkov radiation much like a sonic boom in air. The pattern of light on the tube array reveals the neutrino's energy, direction, and type. Hyper-K will study neutrinos from the Sun, from supernovae, from radioactive decay inside the Earth itself, and from a man-made beam fired 295 kilometers across Honshu from the J-PARC accelerator complex on the Pacific coast. One of its most tantalizing goals is detecting the diffuse glow of neutrinos left over from every supernova that has ever exploded in the history of the universe -- particles never yet observed by any instrument.

Construction began in earnest after the Japanese Diet approved the project in February 2020. Workers bored a new access tunnel into Nijuugo Mountain and began excavating the main cavern 600 meters below the summit, directly above the Tochibora mine where Super-Kamiokande already operates. By October 2023 the dome section was complete, and in July 2025 the full excavation was finished -- 330,000 cubic meters of rock removed to create a void large enough to swallow a 20-story building. Mass production of the new 50-centimeter photomultiplier tubes began in 2021. The next phase transforms the raw cavern into a precision instrument: a two-layer detector structure will be built inside, dividing the tank into an inner detector and an outer detector that screens out cosmic-ray muons. Equipment installation is scheduled for completion in 2027, followed by the slow fill of ultra-pure water. First data is expected in 2028.

Hyper-Kamiokande's scientific ambitions are staggering in scope. If protons do decay -- as many theories of physics beyond the Standard Model predict -- ten years of observation could push the known lower limit on proton lifetime from 1.6 times 10 to the 34th years to 6.3 times 10 to the 34th years, timescales that dwarf the age of the universe by a factor of a trillion trillion. The experiment will also search for the matter-antimatter asymmetry encoded in neutrino oscillations, potentially explaining why the universe contains matter at all. It could even detect neutrinos produced by dark matter annihilation near the galactic center. And if a supernova erupts within our galaxy during the detector's lifetime, Hyper-K would capture tens of thousands of neutrinos in a single burst, providing an unprecedented real-time portrait of a star's death.