For decades now, we've heard about tunnels, used to smuggle drugs - and sometimes people- underneath the US-Mexico border.
Just this past weekend, 30 people were detained in San Diego. Authorities suspect they had been smuggled through a tunnel.
Border and Customs Protection is still investigating.
But here's the twist.
Most of them were Chinese Nationals.
Mike Unzueta is a retired special agent for Homeland Security, in charge of Immigration and Customs Enforcement investigations.
He's now a consultant with Frontier Solutions, a crisis management firm.
And he knows his smuggling tunnels. He spoke with Take Two's A Martinez.