J.S. Bach's Toccata and Fugue in D Minor, BWV 565, is perhaps the single most recognizable piece of pipe organ music in the western world - or at least the ominous opening is.
A fully orchestrated, but slightly truncated version serves as the centerpiece of Fantasia's first segment, set to increasingly trippy visuals.
In the Francophone world, it's known for being the theme song for the edutainment show Il était une fois... l'Homme (Once Upon a Time... Man).
There has been some debate as to whether Bach actually wrote the piece at all. The only manuscript of the work that has survived to the present day was penned by Johannes Ringk, who attributes the piece to Bach; this copy also lacks a date, but is thought to have been written somewhere between 1730 and 1735. The work itself contains a number of stylistic anachronisms, such as the large lack of counterpoint in the toccata, the fugue utilizing subdominant rather than dominant answers, and wrapping up on a plagal cadence, among others. (Then again, the later styles had to draw their inspiration from somewhere — perhaps they drew their inspiration from this piece, and Bach actually started it all!) Another theory is that it was written down poorly by one of Bach's students. Another school of thought holds that it may have been originally written for solo violin (possibly by Bach but likely by another unknown composer), and then transcribed by Bach for the organ; several attempts to reconstruct the piece in this manner have been produced. Bach's most recent biographer Christoph Wolff believes that it's definitely by Bach, but that its stylistic oddities can be explained by its being an early work.
Note that Bach actually wrote two sets of pieces entitled Toccata and Fugue in D minor, but the second (BWV 538) is set apart by its toccata being in the Dorian mode (using a key signature usually used for A minor, i.e. no sharps or flats) for the majority of its duration, a component which gives BWV 538 the "Dorian" nickname in the music world. The fugue is, however, in the traditional natural minor scale (Aeolian mode). This piece is, of course, nowhere near as well-known as BWV 565.