The Bielefeld Conspiracy (German: Die Bielefeld Verschwörung) is a satirical conspiracy theory which claims that the city of Bielefeld* in Regierungsbezirk Detmold, Germany is a geographically nonexistent entity and* an illusion perpetuated by a secret organization known only as SIE , which translates to "THEM" in English, and therefore anyone who contradicts the notion is in on the conspiracy. Since its introduction through a Usenet newsgroup in May 1994, the Bielefeld Conspiracy has grown into a classic Internet joke among Germans to mock the absurdity of conspiracy theories in general.
On May 16th, 1994, computer science student Achim Held of the University of Kiel submitted a message to the de.talk.bizarre newsgroup on Usenet, in which he claimed that he and a small group of friends discovered that the city of Bielefeld did not actually exist. Held speculated that a secret society may be behind the conspiracy, which may have infilitrated the German government. According to a 2004 interview with Held, the joke was inspired by a conversation with an avid reader of New Age magaziines at a student party in 1993.
The joke circulated widely throughout the German web over the next several years. On April 1st, 1999, the city of Bielfeld released a press statement in response to the story, titled Bielefeld gibt es doch! (Bielefeld is real!). In 2009, a film depicting the background behind the fictional conspiracy was made by students at Bielefeld University(shown below, left). The movie, titled Die Bielefeld Verschwörung, was a low budget action/thriller, financed by the university itself and several local sponsors from the city of Bielefeld. On May 20th, 2011, an article about the conspiracy was created on Rational Wiki. On November 14th, 2012, the Goethe-Institut YouTube channel uploaded a video explaining the Bielefeld conspiracy (shown below, right).
On November 12th, 2013, YouTuber rewboss uploaded a video titled "10 things you might not have known about Germany," which listed the Bielefeld conspiracy among other interesting factoids about Germany (shown below, left). On June 3rd, 2014, the DW (English) YouTube channel uploaded a video about the Bielefeld conspiracy (shown below, right). On July 24th, Redditor l_v_r submitted a post about the conspiracy to the /r/todayilearned subreddit, where it gained over 3,900 notes and 630 comments prior to being archived.
On April 10th, 2015, Redditor JoNe96 submitted a post titled "Anybody from Bielefeld?" to the /r/germany subreddit, where many users replied that the city does not exist. On July 6th, the news blog ModernNotion published an article about the history of the Bielefeld conspiracy.
 sixtus – Transcript of the TV interview with Achim Held in 2004 (german)