筑波市
K2K is a neutrino experiment which directed a beam of muon neutrinos () from the proton synchrotron at the KEK, located in Tsukuba, Ibaraki, to the Kamioka Observatory, located in Kamioka, Gifu, about 250 km away. The muon neutrinos travelled through Earth, which allowed them to oscillate (change) into other flavours of neutrinos, namely into electron neutrinos () and tau neutrinos (). K2K however, focused only on oscillations.