At the slightest whiff of the unmistakable nuclear odor, these radionuclide detectors will send the alarm to the nerve center. This International Data Centre will have to constantly and efficiently digest the raw feeds, says Steve Bratt, director of the prototype center, and send its readout to each country's scientists for interpretation. But a bigger challenge will be detecting covert explosions-in the atmosphere, at sea, or underground. 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.
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. 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. 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.