The debate in Washington about funding President Trump's border wall comes amid growing questions about a barrier's effectiveness in the fight against drug trafficking. Since 1990, Border Patrol agents have discovered at least 230 cross-border tunnels running from Mexico into California and Arizona, where they've seized loads of cocaine, methamphetamines and marijuana inside them.

Border Patrol agents not only have to protect what's above the border but also what lies beneath with smugglers taking advantage of existing infrastructure.

"This is already in place built by the cities to drain water," explained Kevin Hecht from inside a storm drain. He's the deputy patrol agent in charge in Nogales.

"What they're building is tapping into this and they're building an illicit tunnel," Hecht said.

Hecht took CBS News' Mireya Villarreal through the tunnels used to train agents to spot breaches made by drug smugglers.

"Sometimes we've got to strip down because it's so tight. Gun belt off and gun in the hand and a flashlight in the other … very, very simple," Hecht said.

Earlier this month, the Mexican federal police discovered a tunnel that accessed a sewer system that flows into the United States and just last month Border Patrol sealed an unfinished tunnel that crossed into Arizona.

Hecht said smugglers' ongoing use of them makes tunnel training essential.

"In training you'll go in this pipe and you'll go 'Ok, I'm above ground and crawling in a pipe, everything is fine,' Ok let's put you somewhere under some earth and see how you react," Hecht said.

He said you're either going to get past that claustrophobic feeling or you're not.

"They're smuggling contraband into these pipes … so we need to make that stop, so the point is to find those, remediate the tunnel, fill it with concrete and move on to the next pipe," Hecht said.