There are many interesting sovereign "nations" out there. Monaco. Andorra. Sealand. Hong Kong. They're even more interesting when you've got to deal with them on the same judicial procedural terms that you use with "regular" nations, since the underlying doctrines don't always make sense as applied to these special cases. (I'm thinking, for example, about the rules that used to say that there was no Article III jurisdiction over residents of Hong Kong during the British era because they weren't "citizens or subjects" of a foreign state.)
Among the interesting nations in this wide wide world of ours is the Vatican. Which is fascinating on a number of different levels. But, for present purposes, figuring out how the status of the Holy See interrelates with the complex sovereign immunity provisions of FSIA (particularly in the context of child sexual abuse allegations) is fascinating.
As this opinion by Judge Berzon (and concurrence by Judge Fernandez) amply reveals.