In August 2019, the city of Bielefeld, home to about 340,000 people in northwest Germany, launched a new marketing campaign based on an old internet joke. In 1994, Achim Held, a computer science student at the University of Kiel, had jokingly spread the rumor that Bielefeld did not actually exist.1 Twenty-five years later, the city’s marketing agency put a new spin on the so-called Bielefeld conspiracy by offering a reward of €1 million for proof that Bielefeld, indeed, did not exist. For once, German humor—quite surprisingly to some—attracted attention far beyond national borders: Entries arrived from participants as far away as China, India, and Australia. Their purported proofs used arguments from such diverse fields as history, physics, and mathematics. In order to make sense of the more complex contributions, the marketing agency’s jury even consulted researchers at Bielefeld’s university and archives. Somewhat less surprisingly, none of the competitors ended up taking home the prize money.2 Proof of nonexistence, apparently, can be quite a nut to crack.