You ever wondered what happened at the White House, a few moments before the nuke struck? Well, let me tell you this, I found this secret thing...
First, what I had to do, was try and get at least a D on every single level. You know how hard it is to intentionally be bad at the game? Right after Apocalypse, you will have to hold down the buttons W and H. I think these stand for White House, that's the most logical answer. These aren't that hard to do, but they're quite tedious, except of the W and H thing. If you want to play this level for yourself, go ahead and give it a try by doing what I just described!
For further details about how I found this out, you can read this pastebin I put together: https://pastebin.com/zeZj589k