The atmosphere contains many stable carbon atoms and relatively few radiocarbon atoms.
The ratio of radiocarbon to stable carbon atoms in the atmosphere has varied in the past.
However, cosmic radiation constantly collides with atoms in the upper atmosphere.
Part of the result of these collisions is the production of radiocarbon (C, pronounced "c fourteen"), carbon atoms which are chemically the same as stable carbon, but have two extra neutrons.
In 2008, nearly two pounds of still-green plant material were found in a 2,700-year-old grave in Mongolia's Gobi Desert.