A human-smuggling tunnel was discovered on the California-Mexico border and authorities arrested 30 people who illegally crossed over — most of them Chinese nationals, officials said Sunday.

Authorities discovered the 3-mile-long passage after spotting a hole in the ground and a ladder at Otay Mesa port near San Diego at around 1:15 a.m. on Saturday, US Customs and Border Protection officials told ABC News.

The 30 undocumented immigrants — 23 of whom were Chinese and seven Mexican — were arrested.

It’s likely a case of human smuggling, authorities said.

“Subterranean tunnels are . . . more commonly utilized by transnational criminal organizations to smuggle narcotics,” the CBP said in a statement.

“However, as this case demonstrates, law enforcement has also identified instances where such tunnels were used to facilitate human smuggling.”

Before discovering the tunnel, Border Patrol agents encountered several people “who had apparently just been smuggled into the US,” CBP said in a statement.

The tunnel started in a building in Tijuana, approximately 328 feet south of the border, NBC reported.