OTAY MESA, CALIFORNIA — U.S. Border Patrol agents discovered a cross-border smuggling tunnel near San Diego's Otay Mesa port of entry after dozens of illegal immigrants emerged from it on Saturday.

The cross-border smuggling tunnel was found just north of the secondary fence in the vicinity of the Otay Mesa port of entry. The tunnel allegedly began in a building on Calle Mar Bermejo in the Garita de Otay area in Tijuana.

Border agents were patrolling the area at around 1:12 a.m. when they saw a large number of men and women on the street who fled when agents approached them.

According to NBC News, some of the people tried to climb back into the tunnel, leading the agents to the discovery. The tunnel is described as a crude opening in the ground with a ladder inside.

Cross-border tunnels are usually used for smuggle narcotics, but no drugs have been discovered inside this tunnel.

A total of 30 people have been detained. Twenty-three were Chinese nationals and seven were Mexicans.

Customs and Border Patrol officials told NBC News that there has been an increase in nationals from countries other than Mexico trying to enter the U.S. illegally in the last two years.