An agent discovered another tunnel on the US border after nearly falling into it

A US Border Patrol agent came across a depression in the ground along the US-Mexico border while on patrol 2 miles east of the Calexico, California, border crossing.

The agent approached the area - near the All-American Canal - on foot, but the ground gave way, revealing a hole 18 inches in diameter, US Customs and Border Protection said in a statement.

The agent observed lumber and wiring in the hole, and further investigation uncovered a tunnel 3 feet by 3 feet wide that stretched 142 feet across the US border.

The tunnel's point of origin was about 60 feet south of the US-Mexico border, in "an area that is primarily open fields and farmland," according to the Los Angeles Times.

It wasn't clear whether the tunnel was complete, and the CBP did not specify its purpose, but the agency did note that, "Anything or anyone could potentially cross into the US via a tunnel."

Though it's not clear how this tunnel was used, or meant to be used, it is the third tunnel uncovered in the Calexico area in the last year.

In late March, US agents busted a 400-yard cross-border tunnel reaching from a home in Mexicali, Mexico, to Calexico after observing its construction for several months.

The smugglers involved reportedly purchased property on the US side of the border and US authorities intervened when drugs began to move through it.

In April 2015, US agents picked up several suspected smugglers along the US side of the All-American Canal, which runs parallel to the international border.

In that case, US agents subsequently discovered a sophisticated, underwater, cross-border tunnel.

The tunnel was approximately 4 feet by 4 feet and equipped with lighting and ventilation, according to a US Customs and Border Protection report.

Evelio Padilla, a 28-year-old Honduras immigrant who came to the US illegally, was arrested while wearing scuba gear and carrying nearly $700,000 worth of meth.

In August 2015, Padilla plead guilty in federal court in San Diego to one count of possession of drugs with intent to distribute.

According to US Immigration and Customs Enforcement, the tunnels recently uncovered in the Calexico area are some of the first subterranean activity discovered in the region in nearly a decade.

"Calexico is less desirable for tunnels because the soil is denser and more difficult to break, and the town's residential character makes it difficult to conceal tunneling activity," US authorities told CNN after the discovery of the cross-border tunnel in March this year.

Smugglers typically focus tunnel-construction efforts in industrial areas of Tijuana, across the border from San Diego, where the soil is conductive to tunneling and where commercial activity helps obscure their operations from authorities.

Despite these discoveries, however, there are likely other tunnels that have escaped notice.

"The US-Mexico border is literally riddled with tunnels, and the big problem is that there's no technology that can detect those tunnels," Mike Vigil, the former chief of the DEA's international operations, told Business Insider.

For every tunnel found, Vigil said, about 10 go undetected.

Tunnels are "a smuggling method that will probably continue and probably increase in the future, especially with the increased border patrols and the surveillance that is taking place along the border," Vigil said.