Die Drei ??? is a popular series of German audio books based on The Three Investigators, an American series of juvenile detective books featuring characters created by Robert Arthur Jr. The original series of 43 books, which ran from 1964 to 1987, involved a trio of teenage detectives who solve mysteries involving spooky phenomena. The German adaptation proved tremendously popular, especially the audio book versions, which topped 100 volumes and outnumbered the books themselves. The popularity of the series was such that it was adapted for the cinema and released internationally. The German film series began with Das Geheimnis der Geisterinsel and continued with Das Verfluchte Schloss.