Super-Kamiokande is a 50~kiloton water Cherenkov detector located at the Kamioka Observatory of the Institute for Cosmic Ray Research, University of Tokyo. It was designed to study neutrino oscillations and carry out searches for the decay of the nucleon. The Super-K experiment began in 1996 and in the ensuing years of running has produced extremely important results in the fields of atmospheric and solar neutrino oscillations, along with setting stringent limits on the decay of the nucleon and the existence of dark matter and astrophysical sources of neutrinos. Perhaps most crucially, Super-K for the first time definitively showed that neutrinos have mass and undergo flavor oscillations.
This facility is in the Mozumi mine of the Kamioka Mining Company in Gifu prefecture, in the Japanese alps. Super-K consists of two concentric, optically separated water Cherenkov detectors contained in a stainless steel tank 42 meters high and 39.3 meters in diameter, holding a total mass of 50,000 tons of water. The inner detector is comprised of 11,146 Hamamatsu~R3600 50~cm diameter photomultiplier tubes (PMTs), viewing a cylindrical volume of pure water 16.9~m in radius and 36.2~m high.