Within the first microsecond of a subterranean blast, a fiery bubble of vaporized rock forms at pressures of several million atmospheres. The expanding gas forces open a cavity, while the shock wave pulverizes rock as it expands into the surrounding earth, then travel for hundreds of miles through the planet. Seismographic stations feel the waves as they pass beneath.

Seismographic station
The breadbox-sized seismographs are stationed in groups of three, each detecting one type of motion: up-down, north-south or east-west. From the readings, scientists estimate an event's size and location-which they can now do for any magnitude 4.0 event, the seismic equivalent of an unconcealed one kiloton (kT) blast. (The bomb dropped on Hiroshima was 15 kT.)

Ideally, says Peter Marshall, a seismologist with Britain's Atomic Weapons Establishment, they would like to be able to do the same for covert 1 kT blasts.