It's not just that El Mirador, the gigantic 7-mile-wide Mayan city newly discovered in Guatemala, may well be the oldest city in our hemisphere. . . .

Or that its greatest pyramid, with a base nearly 400 feet long, six times as big as the largest known, could prove the grandest man-made structure in Central America. . . .

Or that the exploration of El Mirador (running probably to $3 million over 10 years) will most likely be the last of the great archeological adventures. . . .

No, what interests archeologist Bruce H. Dahlin most of all is that this site, dating from perhaps 1,000 B.C. and supporting at one time as many as 80,000 people, is bound to give us basic insights - ancient ecological lore forgotten for centuries - into using the land and the water and the air without polluting it.

Dahlin, of Catholic University, and Raymond Matheny from Brigham Young University led a team that has been exploring El Mirador since 1978. They hope to go back next January, this time with mapping computers from Archeometrics Inc., which use infrared and microwave techniques to chart buildings in precise detail just as though the trees, vines, lianas and assorted bushes weren't covering them six feet deep.

"It should reduce our surveying time by 90 percent," Dahlin said.

The ruins were discovered in 1965 by Ian Graham, a Harvard archeologist, who could only spend 10 days there because of the lack of water.

Graham, contacted at Harvard, said he first noted the lump in the vast green carpet of Guatemala from a DC3, later checked it out on aerial photos and found not only signs of a temple but an extraordinary number of ancient roads radiating from it like spokes.

Reaching the site with the help of local chicladeros, or chewing gum workers ("Oh, it was exciting, yes, the size of it, but not much to see") he did some mapping and eventually wrote about the site in a Tulane University publication. (The chicladeros by the way, were immensely helpful and friendly to the Dahlin group, coaching them in jungle survival, spotting snakes and water sources and so on.)

Graham, whose speciality is hieroglyphs on ancient sculptures, has discovered a number of sites but is not so concerned with developing them. He did find a few sculptures at El Mirador, and he has returned a few times, but it is not a big thing in his life.

The name El Mirador comes, he said, from the chicladeros.

As a specialist in settlement and subsistence patterns, Dahlin is fascinated by some of El Mirador's mysteries:

With water the great problem, since this monsoon-fed jungle is parched three months of the year, how could such a large population thrive using only reservoirs? There is evidence that surfaces were drained into the reservoirs, but this would collect wastes too, so how was the water purified?

Dahlin has found that a limestone layer lines the reservoirs, neutralizing the acid content and promoting bacterial growth. "This seems to be a complete biologically oriented pollution control system," remarked. "It's very subtle indeed."

Historians have long speculated why the Mayans regularly abandoned cities apparently at their peak, with no signs of war or pestilence. Did farmers exhaust the soil? But the Mayans didn't plant single-crop as we do, didn't exploit their land but protected it in various ways. They even turned swamps into valuable crop and fertilizer sources with their ingenious raised-field system.

So why did everyone walk away from El Mirador in the 6th century, easily 200 years before the Mayan civilization peaked? And why does it have earthworks around it?

"My own scenario is that it collapsed sometime around 380 A.D., because at the time the population of Tikal, 80 miles south, mushroomed to triple its former size. It could have been that the water preservation system broke down at El Mirador. Or it could have been razed in a war with other Mayans."

Certainly the place is a wreck today, Dahlin added. It's not photogenic at all. It's not badly looted, however, only about 75 structures having been hit - out of several thousand.

There's a reason. El Mirador is 40 miles from the nearest village. Dahlin and his wife Andrea, a filmmaker and graduate student in archeology, walked in with 17 others while another group dropped onto the site from helicopters.

There are scorpions, snakes, ticks (200 were taken off one man), tarantulas and a charming parasite called the comoyote which plants larvae under your skin. The larvae grow to the size of garden slugs which have to be cut out of you.

"But we loved that walk in," Dahlin said. "So quiet, so beautiful. We built an airstrip there, but we still want to walk in when we go again."

Nevertheless, looting continues. Even as the Dahlins trekked out, they met a party of hunters coming in. The hunters had no guns.

"There's not much there for the art market. It's very early Mayan, and the 30-odd bowls we found aren't decorated, and the stonework isn't so marvelous."

Camping out on the site, the group lived off deer, wild pig and turkey sho by their hunter. They fetched water on muleback from a well 2 1/2 hours away.

(The party included nine chickens, imported on muleback to eat up the ticks, but the mules hated carrying them and tried to rub them off on trees. Everyone despised the chickens, especially a hermaphroditic one that turned into a rooster and create such turmoil among his colleagues that he died of a heart attack.)

El Mirado (which means Lookout or Vista because of the tremendous 360-degree view from the 200-foot-high temple) could change our concepts of pre-Colombian history, pushing back its dawn by as much as 700 years.

For Dahlin, 38, a veteran of a 1973 dig at Belize and an underwater site off Cyprus, it could also help us make jungles livable and swamps useful: It could change the way we view the land itself. CAPTION: Picture, Bruce H. Dahlin, by Larry Morris - The Washington Post