Cyber-criminals around the world are using Internet-based communication channels to establish trade relationships and complete fraudulent transactions. Furthermore, they control and operate publicly accessible information channels that serve as marketplaces for the underground economy. In this work, we present a novel system for automatically monitoring these channels and their participants. Our approach is focused on creating a stealthy system, which allows it to stay largely undetected by both marketplace operators and participants. We implemented a prototype that is capable of monitoring IRC (Internet Relay Chat) and web forum marketplaces, and successfully performed an experimental evaluation over a period of 11 months. In our experimental evaluation we present the findings about the captured underground information channels and their characteristics.