# Strongest axiomatic systems accepted as consistent by math community

As I understand it, most math is done implicitly within $$ZFC$$, but sometimes stronger systems are used--for instance, the initial proof of Fermat's Last Theorem used Grothendieck universes, which imply the existence of large cardinals.

I am curious how powerful axioms you could use before a proof of some $$\Pi_1$$ statement (so that only consistency, not soundness, need be considered) using them wouldn't be generally accepted by the math community. If there were a proof of, say, the Riemann Hypothesis in $$ZFC+X$$, how weak would $$X$$ need to be for the authors to get the Millenium Prize money?