In Super Mario Bros. 3, you could go behind the solid blocks by holding down while on a white one. By my calculation, that would be Mario being capable of moving in three dimensions: up-down, left right, and some motion (albeit very limited) in screen depth. That would make 1988 the first appearance of 3D motion in a Mario game. SMW also let you go on both sides of those metal grates. This is nothing new!