Alpha decay was one of the first discoveries of nuclear physics: a heavy nucleus can decay with the emission of a nucleus, or alpha particle. In fact, decay of heavy nuclei is the source of terrestrial helium (in particular from oil wells). Uranium, the highest Z naturally-ocurring element, is widely spread in the environment. Alpha decay is an important heat source within the earth (and keeps the core molten). It is an important source of environmental background radiation (e.g. from the rocks at Clifton, from radon seeping into enclosed spaces, etc). Alpha emitters have industrial applications (e.g. smoke detectors).