The other day, the US Border Patrol discovered a tunnel along the Mexican border near San Diego. And the agency arrested 30 people — mostly Chinese — who had crept through the tunnel near the Otay Mesa, San Diego area.

A couple weeks ago, I told you that what is going on south of the border could be one of the dozen things that might shock Wall Street in the fall. (North Korea was another, but that’s an easy call.)

Well, everyone in the US should pay very close attention to this tunnel.

As I’ve said before, John Kelly, the former head of Homeland Security and now Trump’s chief of staff, has expressed concerns about what is happening in Mexico.

And he has specifically said possible attacks on aircraft keep him awake at night.

Tunnels. Possible attacks on planes. Chinese. Let’s hope that Kelly and my source are just being scaredy-cats, but everyone should be paying attention to what’s going on in Mexico — and it has very little to do with Mexican immigrants.

Here’s an idea: Since people are just going to tunnel under Trump’s wall, maybe we should dig a 100-foot-deep moat. And put high-tech monitors along the border.

It’ll be a lot cheaper and faster. And thanks to Hurricane Harvey, Texas now has enough water to fill the moat all the way to the West Coast.

I can’t tell you anymore right now about this potential threat. But believe me, it warrants all of our attention.