"Underground - Super Mario Bros. 2" is a high quality rip of "Underground" from Super Mario Bros. 2
The rip is a 8-bit NES rendition of the song "Bowser's Theme" from Super Mario 64. The tempo is a bit faster than the original theme and the rhythm is more energic.
Note that Super Mario Bros. 2 is the only main Super Mario game where Bowser doesn't appear, due to the game being a modified version of the Famicom game Yume Kōjō: Doki Doki Panic. Instead, the main antagonist is Wart, a toad.