A global network of seismic and infrasound monitoring stations listens constantly for underground clues that a nuclear test has taken place. Set up by the Comprehensive Test Ban Treaty Organization (CTBTO) Preparatory Commission, the stations will be part of the verification system for a comprehensive test ban treaty, should it come into force. The United States signed the treaty in 1996, but in 1999, the US Congress declined to ratify it. Since then, efforts to bring the treaty into force have stalled. Just the same, most countries have observed it, and the monitoring system is widely credited with being able to identify any nuclear tests that are conducted. This video, produced by the CTBTO, uses a monitoring station in Bischofsreut, a tranquil corner of Germany's Bavarian Forest, to explain how the global nuclear detection system works.