"The United States discovered Mexico around 1975," someone says eaustically in Carl Rowan's documentary airing for an hour Sunday at 8 p.m. on Channel 9. He meant our sudden realization that Mexico has gas and oil, lots of it.

For those of us who think of Mexico as a beach or a pop song, or who never think of it at all, "Mexico Challenges Uncle Sam" will be an eye opener. Rowan covers and incredible amount of ground, from President Lopez-Portillo's verbal slap at President Carter on a recent visit to the complexities of the illegal immigration problem.

Working hard to keep the subject visual, the distinguished commentator alternates plenty of zooms and freeze frames and shots of billowing yellow flames as unsold natural gas is flared off into the sky. The talking heads are kept to a minimum.

Perhaps the most impressive single shot is an 1847 map showing that Mexico once possessed New Mexico, Arizona, California, Nevada, and parts of Utah and Texas. It gives a new dimension to modern Mexican resentment of its arrogant big neighbor and to the persistence with which illegals cross the border seeking a better life in territory that once was theirs.

He also touches -- briefly but telling by -- on the bitterness of many North American blacks who fear they will be undercut in job competition, especially if an amnesty is declared.

Contrasts are drawn in his interviews with Americans in gas lines who cry "Unfair!" at Mexico's decision to produce only as much oil as is good for Mexico's economy, and with Mexicans who frankly mistrust our motives and believe we are quite capable of using force in a pinch.

Of course no one is going to convey the the TV audience in one hour, starting from scratch, just what the average Mexican thinks of us, let alone what the rest of the world thinks. But Rowan, who says he boiled down 22 hours of tape, is sure to start a lot of people reflecting on the delicate interdependence of industrial nations. He is planning a sequel.