As old things go in Mexico, the Great Temple of the Aztecs is relatively new--something under 700 years.

And it wasn't as though nobody knew it had been there. The chroniclers of the conquests of Hernando Cortez described it in lavish and graphic detail. And pieces of it have been turning up in Mexico City for years--when the subway was built, when utility lines were laid.

The Spanish, however, had, it was understood, carefully dismantled the Templo Mayor de Tenochtitlan--over a period of about 15 years--step by step, stone by stone, carved figure by carved figure, and, using the Aztecs' own evenly carved volcanic blocks, built in the succeeding decades their own imposing Cathedral that is today the centerpiece of Mexico City's huge central square, the Zocalo. Bits and pieces of the Aztecs were left behind, perhaps, but nobody expected more.

However, the carelessness of the conquistadores, bent upon their wanton 16th-century cultural rape, has left a newly discovered legacy in downtown Mexico City that, when it opens to the public this fall, may yet prove the top tourist hit in a city already rich in archeological wonders, ancient plunder and the historical ups and downs of some 4,000 years--give or take a few centuries.

It is the week before Semana Santa, Mexico's celebration of Easter, a time of vacation, family outings, travel. Mexican tourists are as plentiful as the Norteamericanos or the Europeans. Every park bench, every square foot of grass is crowded with unabashedly demonstrative couples, families picnicking, beggars, purveyers of mouthwatering (but absolutely no-nos for American innards) fresh pineapple, coconut, papaya, mango, corn-on-the-cob-on-a-stick, iced drinks of uncertain origin and potability . . .

The Distrito Federal (Federal District, as the capital is also known) is unseasonably hot and the ever-present haze of industrial and automobile pollution is aggravated by the volcanic ash from the Chiapas volcano eruption that has been an on-and-off affair for a few weeks.

The midday sun beats down relentlessly on the Zocalo, which is, except for Red Square in Moscow, the largest city square in the world. Groups of tourists, or often two or three tourists and a guide, mix with civil servants from the Palacio Nacional or one of the other government buildings around the square. Despite a heavy police contingent, traffic is not to be believed: There are no pedestrians, only targets.

In one corner of the square, next to the Cathedral, there is an unprepossessing corrugated metal wall. At one point, where there is a hinged section, there is also a huddle of people. Some of them appear very angry.

As the hinged section is moved to provide an opening, a few people are permitted inside. Those who are not permitted inside do not dissemble their outrage.

"It is a treasure of the Mexican people," shouts one man (in Spanish). "Why are the Norteamericanos let in and the Mexicans not?"

In fact, most of the people who are let in are Mexicans. A few are not.

For two hours a week, now, certain select outsiders--friends, relatives, associates of insiders, some journalists and a few tourists who appear to have, well, made a sizable contribution to certain persons at the hinged section of wall, have been among the first to see the unearthing of what is both among the most exciting and most frustrating facet of Mexico's burgeoning archeological industry.

The Spanish conquerors did not realize that the Aztecs (following the ways of Indian cultures that went before) built pyramidal temple over temple over temple over temple as often as administrations/rulers/cultures changed.

And not realizing this, the Spanish missed an interior temple, the sacrificial stones upon which a huge toll of Aztec prisoners, warriors and children were slain to propitiate a particularly bloodthirsty family of deities. They also missed the receptacle into which bodies tumbled after their living hearts were cut out, as well as some 6,000 objects that had been offered to the insatiable Aztec gods and goddesses. In fact, they missed an entire substratum of buildings and temples.

Modern Mexican archeologists have known for some time that more lay beneath the Zocalo than tradition held, but the proof eluded them until February 1978, when an electric company ditch digger (identified by the National Geographic magazine as Mario Alberto Espejel Perez) hit what he thought was a rock with his shovel. And by that act, the ditch became a dig.

What Espejel uncovered was a carving of the sister of the Aztec god of war. It was no surprise that he and his colleagues promptly notified the government of their discovery.

"It is," shrugged Templo Mayor archaeologist Francisco Hinojosa, "what is required."

Hinojosa, young but veteran of 10 other digs around Mexico City, leads the still-treacherous way on rickety planks around the Templo Mayor excavation. From a small digging under what had been a parking lot, it has expanded to about 1600 square yards, requiring the condemnation and dismantling of a number of buildings, mostly of indifferent architecture and no significant antiquity.

At one end of the excavation area is an incomplete building that will eventually become the Templo Mayor Museum to house the artifacts that have been discovered.

But today the museum is still a shell, and the artifacts, the carvings of jade and stone, the intricate carved beads of turquoise, the ceramic receptacles, the obsidian, the shells, the mother-of-pearl and turquoise inlaid offerings are on display in the very country that would have scooped them up greedily almost 500 years ago. They are in Spain, part of a tour of the treasure that will take them to New York this summer and then back to the Zocalo when the serpents heads, the jaguar carvings, the steps, the walls all become open for public viewing in September.

In general, Mexico is an archeologist's heaven. There are some 11,000 active digs and countless other sites left incomplete or not yet started. As a rule of thumb, one may assume that where there is a smooth sort of mound on a mountain, there is a temple--probably a pyramid, and who knows what else--beneath. Some regret the slowness of the investigations, but in an allocation of priorities, the problems of inflation, overpopulation, pollution and a grim, pervasive, grinding poverty suggest themselves as having a stronger call on national resources.

Nevertheless, there is a sense of excitement about the unearthing of the great temple. It is expected to draw new floods of tourists and already there are booklets and expensive books on the temple and on the treasures found there available for sale to those few who make it inside the walls. For the archeologists, the excitement is tempered, perhaps, by a sense of frustration.

They have excavated all they can . What is still buried--perhaps, for example, the legendary palace of Moctezuma (Montezuma) himself--lies quite inaccessibly beneath the Cathedral or under the National Palace. There is just the merest flash of covetousness in Hinojosa's eyes as he shrugs once more and sighs and says, "It is finished . . . here."