Not cow burps. Cow burps do not contain helium gas. Helium is formed as a product of natural radioactive decay. It tends to get trapped within the earth in th

Not cow burps. Cow burps do not contain helium gas. Helium is formed as a product of natural radioactive decay. It tends to get trapped within the earth in the same ways that natural gas does. Thus, helium is often present as an impurity in natural gas. This is what the previous commentator was referring to, but of course, CH4 is not actually a very good source of helium.