Ground zero.

With the first nuclear bomb, it was easy to find. Wait for the double flash of blinding light and searing heat to pass, then watch for the rising fireball. With a pair of binoculars, start at the crown of the expanding mushroom cloud, then follow the stalk down to where it touches the ground. Bingo. That's where the nuclear bomb was detonated.

These days, though, there is more to detection than spotting mile-high mushroom clouds. Despite their power, bombs can be evasively muffled in underground caves, or masked by synchronizing them with scheduled mining blasts. Rogue nations who get hold of bombs can test them at sea in international waters, miles from the nearest detector. Sensors have to be sensitive enough to detect the blasts from these distances, through air, water, rock, and soil, yet distinguish them from a constant background of earthquakes, volcano eruptions, meteor crashes, and mining activity. And they have to do it now.