When Bach was barely the age of a college kid today, he heard, or saw the score of, a piece by French organist André Raison where he broidered a series of simple variations on top of a four-bar bass line. Realizing the potential of the theme, Bach extended it to double the length, wrote a set of twenty variations that exploited his at the time peerless organ technique, then added a double fugue for good measure. (A double fugue is a fugue with two primary themes, brought in separately or — in this case — together.)
The end result is one of the most colossal pieces in the entire organ literature. Below it is presented in two parts (again due to YouTube length limitations): the split occurs just before the end of the passacaglia which segues immediately into the fugue.
From YouTube, here is a surprisingly “neat” performance by an organist I never heard of named Bernhard Karrer.
If you are an organophobe, try this orchestral arrangement (part 1: passacaglia – part 2: fugue) by, and conducted by, Leopold Stokowski. Or here is a piano transcription of just the passacaglia (which is quite tricky to play, as there is no pedal keyboard and the pianist would really need an extra hand ;-))