Dr. Masatoshi Koshiba, director of Tokyo University's Proton Decay Experiment. The experiment consists of a 12 metre tank filled with 9,000 tonnes of water and lined with 1,000 50 centimetre photomultiplier tubes. The proton's half life is estimated at 10 to the power 34 years, so the water must contain enough protons to provide an average of one decay event per year, an event which may be detected by the tubes as the particles from the decay cause a visible light phenomenon known as Cerenkov radiation. The experiment is taking place 914 metres underground in a zinc mine below Mt. Ikenoyama to minimise the effects of cosmic rays.