The never-ending supply of radon Radon gas is produced by the natural disintegration of radioactive heavy metals uranium and thorium, which are dispersed throughout the Earth's crust. As the atoms of radioactive heavy metals disintegrate, they change into lighter and lighter radioactive heavy metals until they end up as stable, non-radioactive lead. We will never run out of uranium or thorium. Radioactive decay makes life on Earth possible, because it heats the Earth's core. Radon levels on Earth have not changed since the last Ice Age.
Radon is the heaviest known gas, nine times heavier than air.
It is an aberration - the only gas in the long decay chain of heavy metal elements. Its disintegration begets a decay chain of radioactive heavy metals polonium, bismuth and lead. Radon in the soil gas Dig up the top 6 feet of an acre of land and you will find about 50 pounds of uranium. Uranium and its daughter products radium and radon, are found in nearly all rocks and soils. Thorium, which is even more common, also produces radium.