A rogue nation trying to cover up an explosion could detonate a 1 kT bomb in an underground cave and muffle the blast down to a whisper of magnitude 2.5, the size of a barely-felt quake. Earthquakes, bombs, and volcanoes each have a slightly different seismic wiggle, and usually the three can be distinguished by their seismic fingerprints-but below magnitude 4.0, the vibrations weaken, the fingerprints smear, and seismographs can't tell them apart.