Radon is constantly generated in soil by the radioactive decay of radium. The radon concentration in soil air greatly influences the radon migration into a building. The ratio of the radon concentration in room air to the concentration in soil air typically amounts to 1 to 5%. The indoor radon rate strongly depends on the radon generation in the ground, permeability of the soil, weather conditions (temperature, humidity, air pressure) and last but not least from the method of construction and the building materials.
A map of radon concentrations in soil air is the basis for assessing the regional radon situation: An overview of radon surveys in Europe (Pdf , 17MB)
Besides radon can also be dissolved in ground water and migrates in bedrock.
Information contributed by BfS (http://www.bfs.de/en/ion/radon)