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SR388 Underground, commonly referred to as Ancient Chozo Ruins is a theme originally from Metroid II: Return of Samus. The track is included on the Samus Archives Sound Selection CD that is included with the Special and Legacy Editions of Metroid: Samus Returns, a remake of Metroid II. It also appears in The Metroid Legacy during the Metroid II segment, on the Nintendo GameCube Preview Disc DVD.
In Return of Samus, SR388 Underground is heard in Phase 2, Phase 3, Phase 4, and Phase 7, which features remnants and ruins of the Chozo civilization on SR388. The theme also plays near Chozo Statues, making it Metroid II's equivalent to Silence which is never heard. This song only plays while Samus is inside ruined buildings, while different ambient tracks play when Samus is exploring the caverns containing these ruins.
Samus Returns features three remixes of SR388 Underground. The first remix, named M2 Ancient Chozo Ruins Arr, is a slow, atmospheric remix heard throughout the game, in rooms containing purple liquid where the Chozo Seal has not yet been activated (as well as in Area 8, which has no Chozo Seal). The second remix, named Met2 Ancient Chozo Ruins 2 or Area 3, is comparatively upbeat, heard in the lowest section of Area 3. The third remix, named Met2 Ancient Chozo Ruins, is another slow, atmospheric version that plays in the uppermost section of Area 5.
- The SR388 Underground track included on the Samus Archives Sound Selection CD only covers less than half of the full length song. It is the only notably incomplete song on the album.