While an average of 20,000 quakes above magnitude 4.0 are reported annually, seismologists figure that at magnitude 2.5, that number is 120,000-a formidable string of ambiguous rumblings to sift through. In addition, another hundred thousand chemical explosions are set off by mining operations. While these are often less than magnitude 3.0, determined testers can hide nuclear explosions by synchronizing the blasts.

Researchers Scott Phillips and Craig Pearson of the Los Alamos National Laboratories are compiling more detail of the seismic signatures of all possible events into a library. They hope to profile a unique "nuclear fingerprint" for even very small explosions. Similar work with underwater blasts is also underway.