geometry - Where does $pi^2$ appear spontaneously within Physical Phenomenon and Mathematics Equations?

The term $\pi$ is found to appear in many equations and natural phenomenon; however my question is related to $\pi^2$.

While trying to figure out the reason for some $\pi^2$ terms appearing in certain equalities that I came across, I have a question. And the question is this:

In which all mathematics/physics equation or contexts does $\pi^2$ appear inherently?

-- and (now this second part is merely a follow up question that did not form part of the original query but added later) where that $\pi^2$ term can lend some interpretation of the underlying phenomenon, just like does $\pi$ whereby we can interpret (in most cases i.e.) that some type of circular ambulation in 1 dimension is involved??

As you can understand, the $\pi^2$ term is more complex and does not directly lend itself to an interpretation -- as opposed to $\pi$ which is very intuitive.

Thanks