In the 'grassy' stages of Super Mario Bros. 3 (like stage 1-2, for example) you might notice that there's a dark green patch at the bottom of the screen.
You may be thinking that was an artistic choice, but if you're properly prepared, you'll find that it's more than that.
Enter one of those stages with a P-Wing equipped and fly down the closest pit (being careful to not actually fall in, of course) and you'll find that the dark green stuff is actually a tunnel that runs most of the length of the stage!
Of course, stuff like enemy fireballs that go through the ground can still hit you if you're not careful, if you stop mashing the A button you'll fall off the bottom of the screen and lose a life, and you can't actually finish the stage from down there, you have to be above ground to hit the Card at the end.
But it's a fun novelty to do once or twice.