Since the definition of the Busy Beaver function by Radó in 1962, an interesting open question has been what [is] the smallest value of $n$ for which $BB(n)$ is independent of ZFC set theory.
Source: the first sentence of the abstract of the paper A Relatively Small Turing Machine Whose Behavior Is Independent of Set Theory, with the part in bold added by me (I think it was a typo, they likely forgot the "is").
They proceed to prove that such $n$ is at most 7918.
But $BB(7918)$ is a number, right? So what does it mean for a number to be independent of ZFC?
Bonus question: how much is $BB(7918)$?