Muons are particles which live for 2.2 microseconds then decay. (1.56 microseconds is the half-life)
In 1966 muons were created at CERN by colliding a beam into a target, the muons were placed into a ring traveling at 0.997c. They were seen to live 12 times longer than their lifetime at rest.
Muons are created in the upper atmosphere of the earth, 10 km above the surface, when cosmic rays strike molecules. These muons move toward the surface of the earth at nearly the speed of light. At this speed they can move 2,200 feet or about 700 meters before they decay. But most of them make the entire 10 km trip to the surface of the earth. They do this because their "clocks" are seen to run slow by a factor of 10 or more. So they can travel 10 times further and many then reach the surface of the earth.
Viewed from the frame of reference of the muon, the thickness of the earth's atmosphere seems shortened by an order of magnitude so that the muon sees that it covers just 1 km during its lifetime.
Scientific Explorations with Paul Doherty
8 May 2005