Japanese physicist Masatoshi Koshiba (1926-2020) holding a 50cm photomultiplier tube used in the search for proton decay. Hundreds of such tubes lined a tank containing 9000 tonnes of water some 1000 metres underground in a zinc mine in Japan. Tokyo University's Kamiokande experiment was designed to look for decaying protons. If a proton decays, the charged particles it generates move through the water faster than light, and generate blue Cerenkov radiation. It is this that the photomultipliers detect. Computer analysis then determines whether the event was a decay, or a collision with a solar neutrino. The experiment did not observe proton decay, but Koshiba was awarded a third of the 2002 Nobel Prize in Physics for his work on solar neutrinos. Photographed in 1985 in Japan.