Today I received a particularly interesting achievement by chance:
As far as I know, I had never seen this achievement in any of the achievement tabs. The achievement tab this was listed in was "Wonders of the World", which I was monitoring as I was trying to finish all of them. As it turns out, there seems to be more hidden achievements that aren't listed in any of the tabs until you unlock said achievement.
That said -- what are these hidden achievements? How many are there, and what are their conditions?