There are technologies available that would detect attempts to tunnel under the border wall. In addition to ground-penetrating radar, I would assume something like this will be used:

"ELPAM Electronics Ltd. cutting edge technology provides the ideal system for Tunneling Detection.

The offered system is installed underground and is able to recognize a variety of activities digging, drilling, scraping, jack-hammering, etc., measure their intensity and duration.

The system is stationary and fixed after seismic sensors inserted into the ground. The proprietary detection algorithms intelligently filter out non-threatening vibrations, from traffic on nearby roads and underground subways, in order to minimize false alarms."

elpam.com/products/tunnels-disclosure/