By day, the hills above Tijuana are crawling with young men who have slipped across the border and are heading north. By night, the hills nearly swarm. In the dark, a United States border patrolman watches groups break for the north through his nightscope but can only shake his head that there are so many more border scofflaws than agents to stop them.
The legal border crossing between Tijuana in Mexico and San Ysidro, Calif., is the busiest in the world. Likewise, the underground crossing - the holes in the fence and the places in the hills where there is no fence - is abuzz with activity. These hills are said to be dangerous for the waiting Mexicans, because bandits reportedly rob them frequently.
About 40 percent of the illegal border crossers caught last year along the 2, 000-mile US-Mexican border were nabbed in this one section, less than 30 miles long, stretching inland from the Pacific Ocean.
The pressure against the southern side of the border is growing steadily greater. During the last federal fiscal year, 429,200 illegal crossers were apprehended along this border, breaking all previous records. This year, so far, is running about 12 percent ahead of last year.
For border patrolmen the situation is disheartening. One supervisor, noting he had performed his job well for 15 years, added his conclusion that he had never kept anyone out of the United States who wanted to get in.
Experts put the number of illegal aliens living in the US between 3 million and 6 million. About 45 percent are Mexican, another 23 percent from elsewhere in Latin America. About 2 million undocumented residents were actually counted by the Census Bureau in 1980, and demographers used this figure to educate their guesses as to the real number.
In the hills, young Mexicans congregate on the US side of the border during the day, playing soccer or buying refreshments from a vendor, waiting for nightfall to strike out for cities to the north. They scatter as a border patrolman approaches, taking care not to let him get between them and the safe ground of Mexico.
One young man, standing on American soil within darting distance of a hole in the fence, converses casually with a Border Patrol officer from the far side of the patrolman's vehicle. Why not get a work permit and get an American job legally? asks the agent.
The Mexican youth shrugs. ''It takes too long.''