By the 1990s particle physicists were starting to suspect that the solar neutrino problem and atmospheric neutrino deficit had something to do with neutrino oscillation. The Super Kamiokande detector was designed to test the oscillation hypothesis for both solar and atmospheric neutrinos. The Super-Kamiokande detector is massive, even by particle physics standards. It consists of 50,000 tons of pure water surrounded by about 11,200 photomultiplier tubes. The detector was again designed as a cylindrical structure, this time tall and across. The detector was surrounded with a considerably more sophisticated outer detector which could not only act as a veto for cosmic muons but actually help in their reconstruction.