**Question:** For $n\ge 3$, is there a maximal isometrically-invariant finitely additive extension of Lebesgue measure on $\mathbb R^3$?  

A maximal extension of a measure with some properties (in this case, isometric invariance and finite additivity) is one that cannot be extended to a larger algebra of subsets while preserving these properties.

Ciesielski and Pelc (see 
<a href="http://www.ams.org/journals/proc/1990-110-03/S0002-9939-1990-1027089-7/S0002-9939-1990-1027089-7.pdf">here</a>) proved that there is no maximal isometrically-invariant *countably additive* extension of Lebesgue measure on $\mathbb R^n$.  On the other hand, Banach showed that for $n\le 2$, there is a finitely additive extension of Lebesgue measure to *all* subsets of $\mathbb R^n$, and of course it follows from Banach-Tarski that this is false for $n\ge 3$.  

I am assuming Choice throughout, of course.

[Edit: As Bill Johnson points out in the comments, this is just a quick application of Zorn.  I guess that since this is very much nontrivial in the countably additive case, it didn't occur to me that maybe the finitely additive case is really easy!]