It’s lighter than air, chemically inert (for the most part), and the Earth has been around for 4.7 billion years. In all that time, why hasn’t all the helium bubbled out and away to the upper stratosphere?
New helium is being made through radioactive decay, and some of this gets trapped in helium-tight spaces underground.
Just in case you’re asking this because creationists use the presence of atmospheric helium to argue for a young earth, see Radiometric Dating from The Evolution Evidence Page
Wikipedia can be your friend: Helium.
Thanks for that info! I realize you don’t know me, because i’ve been mostly lurking on these boards for years and not posting much (i read them voraciously, though), but i got quite a good laugh out of your mild insinuation that i might be a YEC. I do have three of them in my family, though, and it does make Thanksgiving conversation rather…strained.
PBear: i did check Wikipedia right before asking, but somehow i missed the paragraph about radioactive decay. I’ll read more thoroughly next time.