Over several years, nearly 10,000 on-site soil radon measurements have been gathered in Poland because some of Poland's geological units are covered with soil having a high radon potential (in this study, "high" means that most of the homes have indoor radon levels above 4 pCi/L and "low" means that most of the homes are less than 2 Ci/L). Many of the measurements come from the Upper Silesian Coal Basin in south-central Poland, which contain extensively mined Upper Carboniferous coal-rich strata that overlie Carboniferous limestone strata, and are overlain by Triassic clastic and carbonate strata. Measurements reveal that most of these Carboniferous and Triassic strata have a medium radon potential. Another study of soils on the glacial deposits that dominate most of north and central Poland shows that these materials have a low to medium potential. A third study of the soils developed on Precambrian and lower Paleozoic metamorphic and igneous rocks of the Sudety Mountains of southwest Poland show that most have medium to high radon potential. This was predicted because the terrane contains several types of granites, mineralized fault zones, and vein-type ore deposits (including mined uranium deposits).