A US Homeland Security Investigations Special Response Team agent photographing the tunnel's exit point in Calexico, California. Thomson Reuters US federal agents have arrested four people and seized nearly 3,000 pounds of marijuana in an operation to bust a drug-smuggling ring that had tunneled under the US-Mexico border, federal prosecutors said on Wednesday.

The smugglers built a house to hide the northern entrance to the 400-yard tunnel in Calexico, the US attorney's office for Southern California said.

The southern end of the tunnel came up in the El Sarape restaurant in Mexicali, Mexico, authorities said.

It is the first smuggling tunnel found in more than 10 years in Calexico, a small city of about 40,000 people some 120 miles east of San Diego, but the 12th discovered along Mexico's border with California since 2006.

The rest of the tunnels were found in the San Diego area.

"The Mexico-US border is like a block of cheese with holes in it, with tunnels across it," author and journalist Ioan Grillo told Business Insider.

In the case of Calexico tunnel, federal investigators had been observing it and its operators since its construction began.

A US Homeland Security Investigations Special Response Team agent processing evidence Wednesday during an investigation of a cross-border tunnel linking Calexico and Mexicali, Mexico. REUTERS/U.S. Immigration and Customs Enforcement/Handout

The smugglers purchased the land last April, and by the time construction wrapped up in December, federal investigators were using wiretaps and watching activity at the house, charging documents said.

"This house and tunnel were constructed under the watchful eye of law enforcement," said Laura Duffy, US attorney for Southern California. "For the builders, the financiers, and the operators of these passageways, there is no light at the end of the tunnel."

Federal agents continued to monitor the house, a separate safe house in Calexico, and four suspects until the first loads of marijuana came through at the end of February.

Calexico is a small city of about 40,000 people some 120 miles east of San Diego. Google Maps

On March 7, investigators seized 1,350 pounds of pot that came through the tunnel as it was being transported through the Los Angeles area, according to federal documents.

On Tuesday, a mother and daughter who were allegedly part of the ring were arrested in nearby Nogales, Arizona. On Wednesday, federal investigators arrested two men in Calexico — one at the tunnel house and one at the safe house — and seized an additional 1,532 pounds of marijuana from the tunnel.

The suspects will be charged with conspiracy to import and distribute marijuana, prosecutors said.

While the tunnel discovered this week was the first one found in Calexico in years, smuggling tunnels are a prominent feature of the criminal landscape on the US-Mexico border, particularly in the US Southwest.

A US Homeland Security Investigations Special Response Team agent looking into a tunnel exit point Wednesday during an investigation of a cross-border tunnel linking Calexico and Mexicali. REUTERS/US Immigration and Customs Enforcement/Handout

"The fact is that that 2,000-mile border is riddled with tunnels," Mike Vigil, former chief of international operations for the DEA, told Business Insider in an interview.

The Sinaloa cartel of Joaquín "El Chapo" Guzmán is credited with some of the most ambitious tunnel-building, including several "super-tunnels," which spanned hundreds of meters and had features like air-conditioning, electric lights, and rails for carts used to move goods.

Just a few weeks after Guzmán's brazen jailbreak in July (which was itself through an elaborate tunnel) authorities in Tijuana uncovered another smuggling tunnel connecting the Mexican city with US territory. It was the 181st such tunnel discovered in Tijuana over the last 10 years.

"The border patrol are constantly filling these up with cement, constantly blocking these things,” Grillo said.

(Reporting by Marty Graham in San Diego; Editing by Curtis Skinner and Simon Cameron-Moore)