Carbon 14 dating process
As the Earth's upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon - carbon 14 (C-14).
The unstable isotope is brought to Earth by atmospheric activity, such as storms, and becomes fixed in the biosphere.
Libby in 1949, and has become an indispensable part of the archaeologist's tool kit since.
As a result it is always undergoing natural radioactive decay while the abundances of the other isotopes are unchanged.