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Under ideal conditions, high-tech tools can reveal the location and structure of underground cavities by measuring subtle changes in the force of gravity, seismic waves and electrical resistance. `It`s a real easy job if you have a bunch of graduate students and you can walk around on the ground taking measurements,` said Antony Fraser-Smith, a Stanford University geophysicist.

Afghanistan today is hardly the place for such field trips. Meanwhile, almost all U.S. reconnaissance tools - including radar and communications interception devices - are built for detecting something. Caves are all about the absence of something.

During the Vietnam conflict, U.S. soldiers faced enemies adept at tunneling. One of the most successful detection techniques involved carefully observing entrances for smoke or body odor.

In the early 1990s, a tunnel apparently dug by North Korea was found hundreds of feet beneath the Demilitarized Zone. Seismic testing and radar confirmed the location. Smugglers also move drugs through underground passages from Mexico into the United States. These have usually been discovered through tips, rather than technology.

In all those cases, however, the cave-detectors were people on the ground. To be most effective, cavern-hunting in Afghanistan would require much the same - while likely employing methods originally designed by geologists to find pipes, oil deposits and earthquake faults.