Ground helium is usually decomposed from uranium-235, thorium and uranium-238 and their unstable decay products. Gas accumulates slowly in the earth's crust For millions of years, a ton of granite containing 10 grams of thorium and 2 grams of uranium produced only 0.5 cubic centimeters (or 0.09 mg) of elements. Only a few minerals rich in thorium and uranium can contain high levels of helium. In addition, most minerals undergo weathering, recrystallization and other processes leading to the release of helium. The released bubbles can be dissolved in groundwater or removed from the atmosphere through mineral cracks and cavities.