system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does helium end up underground if it's lighter than air?

I was surprised to find out that helium is mined like natural gas, and I always wanted to know how on earth they end up trapped underground (like other lighter than air molecules and elements) when they're lighter than air.
gpt
Helium is generated underground when heavy radioactive elements deep inside the earth decay.  One common decay process is for a helium nucleus the break away from the larger nucleus and decrease the atomic weight by 4.

The gas then has a tendency to accumulate with natural gas deposits over time, although much of it does slowly make its way out into space.