Fifty seismic stations around the globe--previously used only to feel the ground for natural ground-shaking events-have been rewired to also sense vibrations from underground blasts. Over 120 other earthquake stations can be called on to clarify unclear readings. "Seismic systems," says Lay, "will be the workhorse to detect [nuclear] events."

The ears of the system are under construction. Sixty listening stations will dot the continents to hear for telltale rumbles in the air. They will sense the bass kicks from blasts called infrasound, so low they are beneath the human range of hearing. Undersea listening posts have begun to feel for shock waves from nuclear explosions in the world's oceans. Four new hydroacoustic stations will join the seven already being used by the US Navy to track ocean impacts of missiles.

The nose of the sentinel is also under construction--80 refrigerator-sized boxes that will sniff the air for radioactive particles and gasses. At the slightest whiff of the unmistakable nuclear odor, these radionuclide detectors will send the alarm to the nerve center.