"Is that it?" My wife leans forward in the passenger seat of our sensible hatchback and points ahead to an 18-wheeler that's hauling ass toward us on a low-country stretch of South Carolina's Highway 125. We've been heading west from I-95 toward the Savannah River Site nuclear facility on the Georgia-South Carolina border, in search of nuke truckers. At first the mysterious big rig resembles a commercial gas tanker, but the cab is pristine-looking and there's a simple blue-on-white license plate: US GOVERNMENT. It blows by too quickly to determine whether it's part of the little-known US fleet tasked with transporting some of the most sensitive cargo in existence.

As you weave through interstate traffic, you're unlikely to notice another plain-looking Peterbilt tractor-trailer rolling along in the right-hand lane. The government plates and array of antennas jutting from the cab's roof would hardly register. You'd have no idea that inside the cab an armed federal agent operates a host of electronic countermeasures to keep outsiders from accessing his heavily armored cargo: a nuclear warhead with enough destructive power to level downtown San Francisco.