This natural variation in the ratio of C in the atmosphere.
Archaeology and other human sciences use radiocarbon dating to prove or disprove theories.
Nitrogen atoms high in the atmosphere can be converted to radiocarbon if they are struck by neutrons produced by cosmic ray bombardment.
The rate of bombardment is greatest near the poles, where the Earth's magnetic field is dipping into the Earth and therefore does not deflect incoming cosmic rays.
An age could be estimated by measuring the amount of carbon-14 present in the sample and comparing this against an internationally used reference standard.
The impact of the radiocarbon dating technique on modern man has made it one of the most significant discoveries of the 20th century.