The museum's underground level features a relaxed play area for families to take a break. (warhol.org)
They first appear in World 1 - 2, the first underground level of the game, though only one of them appears in that level. (mariowiki.com)
Soon, though, these assets are joined by the option to make underground levels, and a choice between Super Mario Bros. and New Super Mario Bros.. (heypoorplayer.com)