If we build a wall, we need to defend against tunnels.

The NYT article has an article today: "As Donald Trump Calls for Wall on Mexican Border, Smugglers Dig Tunnels." But Trump's speech last night did address the tunnel problem:

On day one, we will begin working on an impenetrable physical wall on the southern border. We will use the best technology, including above-and below-ground sensors, towers, aerial surveillance and manpower to supplement the wall, find and dislocate tunnels, and keep out the criminal cartels, and Mexico will pay for the wall.

The Times article acknowledges this part of the speech, but says that "no technology exists to reliably detect the tunnels, and experts say it may be years before such a system is developed."

Technological advances such as ground radar to detect movement, hundreds of high-tech cameras with night-vision lenses and drones flying overhead have drastically transformed border security.... The American government has poured hundreds of millions of dollars into research in hopes of finding a way to detect tunnels, but most of these efforts have ended in disappointment....

Part of the problem in detecting tunnels, say experts like Paul Bauman, a Canadian geophysicist, is the ground itself.... With underground cracks, water tables, tree roots and caves, it is hard to tell what is and is not a tunnel, he said....

“For every tunnel we find, we feel they’re building another one somewhere, and they might get more creative in concealing it,” he said. “Next year, I could find 10. Until there is some device on the market to help us accurately detect them, we just won’t know.”

Tunnel detection is something we need to do — whatever you think of Trump's wall. I don't like the tone of defeatism, if that's what this is. You might be politically opposed to Trump and want to say his wall won't work because there will be tunnels, but whatever you think of illegal immigration, there's a drug crime problem that must be dealt with.

Here's an article in The New Yorker from last year about the building of tunnels: "UNDERWORLD: How the Sinaloa drug cartel digs its tunnels." Excerpt:

Across Tijuana, at bus stations and on busy street corners, [young men] were lured to the warehouse by the prospect of temporary jobs.... Most were laborers from Mexico’s rural interior who had travelled north seeking opportunity.....

Carlos [the overseer] split the men into two groups. Fernando worked the day shift, from 6 a.m. to 6 p.m.; at night, he slept in the warehouse with the rest of his crew. Carlos brought the workers food and made sure no one left the building....

Most of the time, five or six men worked inside the chamber, lengthening it into a tunnel by chipping away at the earth with handheld electric spades and filling sandbags with dirt and rocks. Three other workers hauled the bags out using a makeshift elevator—a large metal cage connected to an electric pulley system. The sandbags were then piled onto wooden pallets in the loading bay. Occasionally, Carlos was joined by other overseers, who wore ski masks. They’d threaten to beat the workmen if their northward progress slowed. The workers gained about five metres a day....

The walls retained their form as the men worked, but threats were ever present. The history of subterranean excavation, from the ancient Egyptians to the coal miners of Appalachia, is dense with tragedy—any strike of a pickaxe can release a deadly rush of groundwater, spark a methane fireball, or disrupt the soil enough to cause a collapse....

Joseph DiMeglio, the head of the Tunnel Task Force, told me that, when a tunnel is finished, diggers are sometimes recaptured and forced to work on another project. Other times, he said, “the cartel takes them out back, you know, and gets rid of them.”