Mexican police smashed their way into a narco tunnel stretching across the US-Mexico border

In late March, US federal officials shut down a smuggling tunnel stretching more than 400 yards from Mexicali, Mexico, to Calexico, California, arresting four people and seizing more than 1,500 pounds of marijuana.

The tunnel's entrance on the Mexican side of the border was under the floor in El Sarape restaurant, and the opening on the US side was found under a tile floor in the front room of a three-bedroom home at 902 E. Third Street, located about 300 yards from the border.

In the video below, posted by Baja California-based magazine Zeta, Mexican federal police are seen smashing through the tile floor of the restaurant in Mexicali, pulling out the plug used to seal the entrance, and inspecting the tunnel.

While tunnels have frequently been used by Mexican traffickers to move large quantities of drugs into the US (including marijuana seized from a stash house, the total haul from this tunnel was nearly 3,00o pounds of weed), the tunnel uncovered in Calexico was unique for several reasons.

According to US prosecutors, it was the first known instance of smugglers purchasing property on the US side of the border for the specific purpose of concealing a narco-tunnel entrance.

"The search warrant affidavit and charging documents allege the traffickers scouted properties in the area and selected the Third Street parcel in a residential section of Calexico," the US attorney for the Southern District of California said in a statement.

"The property sale was finalized in April of 2015 for $240,000 by the drug traffickers."

This tunnel was also the second of three discovered over the last year in and around Calexico, an area that hadn't seen tunneling activity for nearly a decade.

In April 2015, men were observed around the All-American Canal, appearing to bring drugs into the US. When authorities responded, they caught a man in a wet suit and scuba gear, as well as $700,000 worth of meth. Further investigation revealed a partially submerged tunnel stretching from Mexicali to the US border.

In April this year, a US border agent came across another tunnel constructed between Mexicali and Calexico when the ground collapsed near the All-American Canal.

That tunnel was found to span 142 feet from the US side of the border to "an area that is primarily open fields and farmland" on the Mexican side, according to the Los Angeles Times.

US authorities did specify what the tunnel's purpose was, and it was not clear whether it was completed, though "Anything or anyone could potentially cross into the U.S. via a tunnel," the US Customs and Border Protection agency said in a statement.

In past, smugglers using tunnels have focused on the Otay Mesa area between Tijuana and San Diego, where the soil is more conducive to tunnel construction and where commercial and industrial activity makes it easier to hide building activity. 

"Calexico is generally considered a less desirable place to construct tunnels because soil composition is more difficult to penetrate, and because it is a largely a residential city, making tunnel exits and smuggling activity more difficult to conceal," the US attorney said in its statement.

The tunnel uncovered in late March was the 12th large-scale operational drug smuggling tunnel found along the California border since 2006, according to the US attorney.

Over the last five years, federal authorities have detected more than 75 cross-border smuggling tunnels, most of which have been found California and Arizona.

However, those may just be a fraction of the tunnels that riddle the US-Mexico frontier. 

"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.

The use of tunnels will likely continue in the future, "especially with the increased border patrols and the surveillance that is taking place along the border," Mike Vigil, author of "Deal" and the former head of international operations at the Drug Enforcement Administration, told Business Insider.

For every tunnel found, as many as 10 could go undetected, Vigil said.