Muon decays in the Earth's atmosphere, time dilatation and relativity of   simultaneity
  Observation of the decay of muons produced in the Earth's atmosphere by cosmic ray interactions provides a graphic illustration of the counter-intuitive space-time predictions of special relativity theory. Muons at rest in the atmosphere decaying simultaneously are subject to a universal time-dilatation effect when viewed from a moving frame and so are also observed to decay simultaneously in all such frames, whereas the decays of muons with different proper frames show relativity of simultaneity when observed from different inertial frames. 
J.H.Field