It was the Mexican police that found the 1/3 mile long smuggler's tunnel connecting a building in Tijuana to a location in San Diego. The find was due to an anonymous tip, according to Daily Mail with the derisive headline Can't build a wall against this! Mexican police find massive tunnel leading from Tijuana to San Diego that is equipped with rails so drugs can be smuggled into the US by the CARTLOAD.
No, a wall on the surface wouldn't be do it. But why isn't there technology available to find abnormal conditions underground? Well, there is, it's just not perfect.
There's seismograph works sometimes. But geological anomalies can make the walls of the tunnel hard to spot.
Then there's ground-penetrating radar which looks like the better of the two.
In any event, every time we hear of a tunnel like this being located we have to wonder how someone could dig a tunnel 1/3 mile without detection. Then again, the article said there was an anonymous time, so someone one noticed. And there's probably no better source of information than a witness who isn't afraid to talk.