Detecting the Smuggling Tunnels Under the Border
Ground Penetrating RadarSource
• How It's Done: GPR uses pulses of radio-frequency energy to see beneath the surface. In commercial use since the 1970s, it is today's standard for detecting voids such as caves and tunnels.
• How It's Done: The way that vibrations just under the surface change as they pass through rock and dirt provides details about what's below, and can show the presence of a tunnel.
• How It's Done: Electrical currents can't leap across empty space at low voltages. Metal electrodes staked in the ground could form a remotely monitored system that would tell solid rock from a void.
• How It's Done: When underground soil is removed, it causes very subtle changes in the Earth's gravitational field. Lower gravity readings can indicate a tunnel.
• How It's Done: Muons are subatomic particles created by cosmic rays hitting the Earth's atmosphere. The number of them detected underground varies with the mass above: If there's a tunnel, more muons are found.
More information about each technology at the linked source above. Some methods work from the air. Most work using sensors embedded in the earth. The cosmic ray method requires placing your muon sensors below where suspected tunnels may be.
Tunnels that run from a building on one side of the border and exit from a building on the other side of the border, have been some of the toughest tunnels to find. Urban development and activity can make some types of sensing more difficult. Tunnels that exit within neighborhoods that are typically "no-go" for authorities, can further hamper detection efforts.
The monetary rewards are quite large for smugglers who can successfully build and maintain cross-border tunnels that allow large throughput of merchandise, with low detectability. Criminal cartels can recruit clever mining engineers and tunnel designers, and bribe officials to look the other way--particularly on the Mexican side, but also on the US side.