Ah, the Mushroom Kingdom. Most Smash players will recognise this place from previous games in the series, where the world was represented in a pixel-art style reminiscent of the original Super Mario Bros.
But why the "y" on the end of "Mushroom"? What’s the difference?

Oh, so that’s why!This time around, we’ve designed a stage that recreates the layout of World 1-1 in Super Mario Bros. – but with rich backgrounds added to the mix.

The stage scrolls forward at a leisurely pace.

Pieces of familiar scenery scroll by.The kingdom you once knew has turned to ruins over the long years, and it is now your battlefield.
You’ll get to enjoy a wide range of familiar structures. Also, since the stage slowly scrolls sideways, battles here will require you to come up with a very different strategy for winning.
Oh, and when you select this stage, it will occasionally load an alternate underground stage!

This is the famous World 1-2. This one’s also quite charming."Super Mario Bros.: Ground Theme" (1:18)
The Mushroomy Kingdom stage sports the most famous stage design in all of gaming.
And the world-famous Mr Koji Kondo, creator of the music for Super Mario Bros., has kindly done us the service of arranging this theme for the stage!
The original overworld theme in Super Mario Bros. had a certain almost-Latin flavour to it, but this time the theme features a piano arrangement that really goes along well with the atmosphere of Mushroomy Kingdom.
It’s just wonderful.
Mr Kondo, thank you for your help!