As Donald Trump calls for wall on Mexican border, smugglers dig tunnels
The New York Times - 09/01/2016
Carey M. Rappaport, a professor of electrical and computer engineering at Northeastern University in Boston, said the depth of many tunnels also posed a technological challenge. Some can be as deep as 90 feet, beyond the reach of most ground-radar devices and sensors.
“Soil is very good at keeping secrets,” said Mr. Rappaport, who has also worked with the United States and Israeli governments on tunnel-detection methods.