Mexican authorities in the border city of Tijuana found a tunnel designed to smuggle people without documentation into the United States, the government said.
Three suspected people-traffickers were arrested, a statement from the National Migration Institute said.
It gave no details on the size of the secret underground passage.
Police acted on an anonymous tip about a possible kidnapping of migrants held in a residential area of Tijuana, which borders California.
Tunnels of varying degrees of size and sophistication have been found in Tijuana. Authorities say some were used to smuggle both people and illegal drugs into the United States.
In one of the most elaborate ones, found in October of last year, police came across eight tons of marijuana and 148 kilos of cocaine.
It was 200 meters long and was equipped with electricity and ventilation, officials have said.
