Proton decay experiment to determine the ultimate stability of matter..View of the entrance of Tokyo University's Proton Decay Experiment. 1,000 50-centimeter photomultiplier tubes line the 12-meter deep tank of water form the experiment. The water contains enough protons to provide an average of one decay event per year, an event that may be detected by these tubes as the particles from the decay cause a visible light phenomenon known as Cerenkov radiation. The experiment is taking place 914 meters underground in a zinc mine below Mt. Ikenoyama to minimize the effects of cosmic rays. Japan. (1985).