The Universe is only about 1.3 × 1010 years old, so radionuclides which were created along with the stable nuclides and which have a half life of 108 years or more are still to be found. The radioisotopes which fall into this category are collected in Table 2.1, along with approximate values of their half-lives. Some of these radionuclides occur together with their stable isotopes in many parts of the natural world, including (in the case of 40K) our own bodies. Others, for which there are no corresponding stable isotopes, occur in mineral deposits or seawater, and provided man with his first examples of radioactivity.