It's not the one you're thinking of (that one's coming later). It actually happens at the very beginning of the game, so there's truly no worrying about spoilers.
The game begins with a plane crash.
Upon surviving the initial impact, you try to escape the wreckage, then you see it.
|That.....doesn't look like it belongs in the middle of the ocean.|
As the bathysphere descends to the ocean floor, Rapture's designer (and the game's antagonist) Andrew Ryan lays out the reasoning behind the city, all the while, giving you a beautiful view of the outside of said city as Garry Schyman's haunting score underlays the whole scene.
The stage is set... can the rest of the game possibly live up to the impressively high bar set in the game's opening ten minutes?
For those who missed out on the game, or for those who simply want revisit Rapture again, here's the game's awe-inspiring first ten minutes.