About 50 years ago, Williard F. Libby, a Professor of Chemistry at the University of Chicago, predicted that a radioactive isotope of carbon, known as carbon-14, would be found to occur in nature. Since carbon is fundamental to life, occurring along with hydrogen in all organic compounds, the detection of such an isotope might form the basis for a method to establish the age of ancient materials. Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer. In contrast, methane made from petroleum products had no measurable radioactivity.

This discovery meant that there are three naturally occurring isotopes of carbon:

• carbon-12 (C-12), comprising 99% of carbon atoms
• carbon-13 (C-13), comprising about 1% of carbon atoms
• carbon-14 (C-14), represented by one carbon atom per trillion

Whereas carbon-12 and carbon-13 are stable isotopes, carbon-14 is unstable or radioactive.

Further reading:

Web-Info radiocarbon dating
http://isotopes.lbl.gov/isotopes/toi.html [click on C in the periodic table]