How well a Comprehensive Nuclear Test Ban Treaty (CTBT) can be verified has been debated for many decades. Verification was of major contention in 1999 during the U.S. Senate debate on ratification. Then-governor George Bush claimed in 2000 and again in 2002 as president that the treaty, which the U.S. signed in 1996, is not verifiable. Since 1995, the international CTBT Organization focused on detecting and locating globally many small seismic events. While its staff filters out many obvious earthquakes, the identification of nuclear explosions and problem seismic events is reserved for national CTBT agencies. Since the findings of national authorities typically are classified, it is difficult to ascertain how well seismic events can be identified. Nevertheless, only a tiny fraction of events detected over the past 42 years were singled out in unclassified scientific and governmental documents or the media as problem events whose identification potentially compromises the verifiability of a CTBT.