This paper presents MIDAS, a 3D Visual Language for designing multimedia active documents. MIDAS represents multimedia documents' structure and behaviour using a compact tri-dimensional representation. It uses Petri nets to describe the dynamic behaviour of documents, while maintaining a simple and intuitive visual representation. Thus, MIDAS provides a simple means for computer aided design of multimedia documents combining time dependent information, such as movies, sound clips and interactive presentations. We discuss the rationale for the design of this new language, provide an overview of MIDAS and describe its salient features through simple examples.