SAN YSIDRO, CALIF. — DUSK along the border. A well-worn path leads through scrub from Mexico into the United States. As the sky darkens, a Mexican in his early 20s appears over the crest of a rocky hill. The man scans the area quickly. Seeing no US Border Patrolmen, he stumbles down a 20-foot embankment, runs across a narrow country road, and vanishes into a thick wood in a San Ysidro suburb.
Minutes later, other illegal aliens hurry along the same path into the US. Some lug suitcases. Others tote shopping bags filled with belongings. They are of all ages. A young man, carrying a child on his shoulders, puffs down the hill. An elderly woman, escorted by a friend, is led carefully along the path. A family - husband, wife, son - scamper into the trees.
Within 20 minutes, a reporter counts 87 illegal aliens along this one path. Later, as night falls, another group of 53 uses the same trail to evade the US Border Patrol.
Immigration experts say a flood tide of illegals once again is pouring across the California border. They can be seen here every night - people from many countries spilling unchecked into the US in groups of 20, 50, even 100 at a time.
On an average night, the Border Patrol station here in San Ysidro apprehends 856 illegals. But many more than that escape every day - and find their way to Los Angeles, San Francisco, Seattle.
California leads the nation in illegal immigration. But experts say the problem is once again worsening everywhere, from Arizona to Florida, from Texas to New York.
In San Ysidro, acting chief agent William Veal of the Border Patrol admits that as many as two out of three illegal entrants get past US guards.
``We're not a very viable deterrent,'' he admits. ``We see a lot more [illegals] than we have officers to catch them.''
He concludes: ``We are drastically underresourced.''
It is no different in Texas. Silvestre Reyes, chief patrol agent for 280 miles along the Rio Grande, says:
``If someone is willing to keep trying, I think the chances are better than 50-50 that they eventually will get through us.''
In El Paso, acting chief agent Gus de la Vina says: ``There's still a large volume [of illegals] moving through our area. I think we probably catch one out of three - that would be a good guess.''
Nationwide, the number of illegals entering the country declined for awhile after passage of the Immigration Reform and Control Act of 1986. The act made it illegal to hire undocumented foreigners in the US - and supposedly eliminated the job magnet that was drawing millions unlawfully to the US.
The peak number of arrests came in 1986 when there were 1,615,854 apprehensions along the Mexican border. Subsequently, the flow of illegal aliens slowed, and arrests dropped to 1,122,067 in 1987, and 943,063 in 1988.
But in 1989, the trend reversed once again. In May, the year-to-year apprehension rate began rising. Arrests that month were up 17 percent over the year before.
At first, officials thought the uptick might be a one-time event. But it was no fluke. In June, 1989, arrests jumped 19 percent; in July, they were up 32 percent; August, up 19 percent; September, up 36 percent; October, up 21 percent; November, up 19 percent.
The illegal-alien problem had returned. The word apparently had gone back to the villages and cities of Mexico and Central America: It is safe again to cross the border. The law against hiring undocumented workers isn't being strongly enforced by the US Immigration and Naturalization Service (INS).
Arthur Corwin, an expert on immigration policy, says the growing crisis is even worse than Border Patrol numbers indicate.
Dr. Corwin says the big drop in arrests after 1986 came about because the new law legalized millions of Mexicans who had unlawfully been going back and forth across the border. It didn't slow the border traffic - it just legitimatized part of it. Corwin has researched immigration policy since 1969, and currently is writing a book on the subject.
Corwin, interviewed in Santa Fe, N.M., says there have been two significant developments in recent months.
First, illegal aliens are becoming more sophisticated. Many of them have relatives in the US. To assure safe passage, more of them are hiring professional smugglers to get past the Border Patrol, or they are simply flying into the US as tourists - then vanishing into large ethnic communities where the INS cannot find them.
Second, the nature of illegal immigration is changing. For many years, most aliens came to the US to work for a few months, or a few years, then returned to their native countries. Today, most of the traffic is one-way. When aliens get here, they stay. Eventually this will lead to huge increases in the number of illegals here.
What can be done? Border Patrolmen from Texas to California have one plea: Send us more help.
In California, Mr. Veal says it would probably take a major increase in manpower to stem the tide. He currently supervises a staff of about 700 agents. He thinks it would take two or three times that amount - 1,400 to 2,100 - to become an ``effective deterrent.''
In Texas, agents say they need about twice as many patrolmen to meet the crisis.
The Center for Immigration Studies, in a November study, concludes that in spite of the 1986 law, illegal migration is rising because of three factors:
1. Rapid growth in the working-age populations of Mexico, Central America, and the Caribbean.
2. Weakening of the 1986 law through use of false documents and other forms of subterfuge.
3. Shrinking INS resources, including fewer Border Patrol agents, due to tight budgets and diminished interest in Congress and the White House.
Experts say the impact of illegal immigration will be greatest in a few major states: California, New York, Texas, Florida, New Jersey, and Illinois.