"Life shrinks or expands in proportion to one's courage"
Congratulations Curious Mind!
Your curiousity has brought you to one of the most insightful portions of Mario's work, the hidden element. In all of Mario's art he has things hidden that you would never notice on casual glance. Once you really start taking a closer look you may notice some of these things. Some are hidden and obscure, others are in plain sight but easily overlooked or not understood.
Here are some examples with some explainations but you'll want to take your own closer look.