I'm always the last person to find out about stuff on the internet, so obviously this comic ran seven months ago.http://xkcd.com/1110/
What one might not guess from first looking at this strip, and what I believe I missed back in September, was that you can in fact Click and Drag that fourth panel to reveal more.
A lot more.
Really, a LOT more.
I would advise that if you are a patient person you should explore the comic like it was originally designed to explore, by clicking and dragging. It will take you a long time. If you go to the discussion forum there are multiple representations of the entire fourth panel. This onehttp://clickanddrag.azurewebsites.net/
allows click, drag, and zoom, and has the parts of the sky where there is content highlighted. And thishttp://xkcd.kajakklubben.org/
is a game where you can use the space bar and arrow keys to move the little man with the balloon around the fourth panel.
I don't think I've ever seen a webcomic that was this poetic.