The earth contains a great many natural radioactive elements, such as uranium, thorium and potassium.
Uranium, for example, is present in all rocks, and in particular granite rocks. When it decomposes, it gives rise to a radioactive family, ultimately forming lead, which is stable. Radon is one of the radioactive decay products of uranium. Its distinguishing sign is that it is a gas.
And because it's a gas, it escapes and accumulates in caves or galleries, which are enclosed spaces. During the first few years when French uranium deposits were mined, the miners breathed air in which the radon content could be as high as 20,000 Becquerels per m3.
Epidemiological studies on uranium miners have shown that radon is a carcinogenic agent that can cause lung cancer. More recently, studies on the general population have confirmed this risk for exposure to radon in the home.