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Tourian (ツーリアン), also known as Tourian (Base Area) (ツーリアン (基地ステージ)), is the theme of the area of the same name in Metroid, Super Metroid and Metroid: Zero Mission. It was originally composed by Hirokazu Tanaka.
In the original game, this theme can also be heard while fighting Kraid and Ridley, effectively making it a boss theme. This is no longer the case in later games, where Kraid and Ridley have their own boss themes: Big Boss Confrontation (Kraid, Crocomire, Phantoon) and Vs. Ridley, respectively, although it continues to play during the first phase of Mother Brain's battle in Super Metroid.
In Metroid and Zero Mission, the theme has a very mechanical sound to it, with a series of high-pitched notes in the original interpreted as electronic beeping in the remake. Its low melody is very dark and sinister, fitting the cold and unforgiving corridors of Tourian. The Zero Mission version is number 16 in the same game's Sound Test.
For Super Metroid, Tourian was remixed by Minako Hamano. This version adds a layer of bubbling sound effects to the theme, fitting the significant amount of lava in Tourian. The melody is softer than in other versions, sounding like it was performed on metal pipes. Due to long periods of no melody, during which the only sound is the bubbling, the song has an "emptier" feel compared to other arrangements.
In the Symphonic Legends concert, WDR Radio Orchestra performed Suite: Samus Aran - Galactic Warrior. A mournful choir rendition the Tourian theme is one of the few recognizable Metroid melodies heard in this arrangement.
A techno, sped-up remix of the theme is heard in Area 8 of Metroid: Samus Returns, within the caverns housing the final ten Metroid larvae. This appearance of the Tourian theme in this remake (replacing a different theme from the original game) effectively cements it as the series's definitive theme for battling Metroid larvae. Compared to other renditions, this one has a very fast-paced and "violent" feel, embodying the aggressive and relentless nature of Metroid larvae. The percussion and ambience in this remix is evocative of the Impact Crater theme and Metroid Prime Battle. It continues to be heard until Samus drops down into the Queen Metroid's chamber, although it will play again if Samus revisits these caves. In the internal files, it is named Area10 Basic.