Editing Hubbert peak theory (section)

===Helium===
[[File:US Helium Production and Storage 1940-2014.png|thumb|Helium production and storage in the United States, 1940–2014 (data from USGS)]]
Almost all [[helium]] on Earth is a result of [[radioactive decay]] of [[uranium]] and [[thorium]]. Helium is extracted by [[fractional distillation]] from natural gas, which contains up to 7% helium. The world's largest helium-rich natural gas fields are found in the United States, especially in the [[Hugoton Natural Gas Area|Hugoton]] and nearby gas fields in Kansas, Oklahoma, and Texas. The extracted [[Helium storage and conservation|helium is stored]] underground in the [[National Helium Reserve]] near [[Amarillo, Texas|Amarillo]], [[Texas]], the self-proclaimed "Helium Capital of the World". Helium production is expected to decline along with natural gas production in these areas.

Helium, which is the second-lightest chemical element, will rise to the upper layers of Earth's [[atmosphere]], where it can forever break free from Earth's gravitational attraction.<ref>{{cite journal
|title=Helium in the Terrestrial Atmosphere
|author=Kockarts, G.
|volume=14| issue = 6
|bibcode=1973SSRv...14..723K
|year=1973
|pages=723ff
|journal=Space Science Reviews
|doi=10.1007/BF00224775
|s2cid=120152603
}}</ref> Approximately 1,600 tons of helium are lost per year as a result of [[atmospheric escape]] mechanisms.<ref name="techdaily">{{cite web|url=http://scitechdaily.com/earth-loses-50000-tonnes-of-mass-every-year/|title=Earth Loses 50,000 Tonnes of Mass Every Year|work=SciTech Daily|date=5 February 2012 }}</ref>