The conclusion that this happens was strengthened when the Super-Kamiokande experiment was able to detect electron and muon neutrinos that originate from cosmic rays that strike the earth's atmoshpere. They found that many of the muon neutrinos oscillated into another kind of neutrinos (probably tau neutrinos).
Updated 22 Octobber 2007
Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA