"Sea Within a Sea" is the first single from UK band The Horrors' second album, Primary Colours. First released online on 17 March 2009 after a week-long countdown on the band's website, the song is notable for its nearly eight-minute length and for not featuring any definable chorus or refrain. The track was produced by Geoff Barrow of the English trip hop band Portishead.
Within one week of its release, the track had clocked over 13,000 plays on the Horrors' MySpace page. The band allowed fans to download an MP3 of the track for free from their official website.
NME magazine described the song as "an eight-minute Spacemen 3-meets-Neu! odyssey of ominous motorik rhythms, Faris' mournful incantations and an expanding starfield of synths." In October 2011, NME placed it at number 101 on its list "150 Best Tracks of the Past 15 Years".