: Written in the key of D Major . The intro specifically starts on an F# note.
To understand the foundation of "Underbase," it helps to know the original's structure: Underbase: Fallen Down
: Like many SiIvaGunner rips, it subverts expectations by presenting what looks like a standard game track but replacing the instruments or melody with a humorous or unexpected reference. Reference for "Fallen Down" (Original Track) : Written in the key of D Major
: The primary melody is based on "Fallen Down," the fourth track of the Undertale OST, originally composed by Toby Fox. Underbase: Fallen Down
If you are looking to play the theme yourself, several resources provide tutorials and tabs: Fallen Down | Undertale Вики | Fandom