UNCOVERING COBA : Half hidden amid the tangled growth of centuries, the ancient Mayan city offers not only the tallest pyramid in Yucatan but also the chance to explore an archeological wonder still unspoiled by crowds of tourists

Living in a city with its smog and its traffic, one forgets about clean air and space--and butterflies. Especially butterflies.

But in the jungles of the Yucatan peninsula, butterflies-- mariposas en espanol --swarm everywhere, fluttering aimlessly across country roads like troupes of graceful ballerinas. And here in Coba, once one of the largest and most splendid of the great Mayan cities, amateur archeologists, with no more effort than a 10-minute hike, can see scores of the brightly colored creatures at certain times of the year.

The jungles of the Yucatan can be unbearably hot and humid, but sighting these spectacular butterflies alone makes for a worthwhile journey. Coba has far more to offer than just mariposas , however.

It takes planning to get here, though. For starters, one needs a rental car (traveling from Cancun or Merida), a full tank of gas and a willingness to venture a few steps off the well-beaten tourist path. Coba is about 90 miles from Cancun, about 150 miles from Merida.

It’s also wise to bring along a can of extra-strength mosquito repellent, and it helps to have a built-in tolerance for heat. But the rewards--the butterflies, the solitude, the intense jungle silence and the tallest pyramid in the Yucatan--are worth any inconvenience.

Coba is not Chichen Itza. The ruins don’t measure up--not in size, nor scope, nor perfection. No one on holiday in the Yucatan should miss Chichen Itza. But to its credit, Coba is free of Chichen Itza’s almost Disneyland-like atmosphere. Missing are the busloads of loudly dressed tourists milling in hordes around the ruins. In Coba, you are generally on your own when you climb a pyramid. Once at the top, you can survey the jungle alone.

In fact, the tenor of your Yucatan vacation changes dramatically as soon as you turn off the main coastal highway that carries tourists from Cancun to Tulum, a distance of about 90 miles. From Tulum and its ruined walled fortress on the turquoise Caribbean, a two-lane, freshly paved road moves northwest into the thick Yucatan jungle. Hundreds of cars and buses jockey for position on the main highway, but during the hour or so that it takes to travel the 35 miles from Tulum to Coba, traffic is sparse.

Alongside the road, in an occasional clearing, mud-and-straw-hut villages house the descendants of Indians who built the great Mayan cities more than 1,000 years ago.

Otherwise there are only tangled trees and impenetrable scrub. Occasionally a pig or a mule will saunter slowly across the road, but otherwise there is little sign of life. En route to Coba there are no gas stations and no telephones. Not even a Coke machine.

Just when you begin to feel that perhaps the map and the tour books are incorrect, that you may be on the wrong road, you come upon the sign that reads: Coba.

“Thank God,” you’ll probably say with a sigh. “Civilization.”

Well, sort of.

In the village of several one-story stucco buildings, entrepreneurs sell cheap souvenirs, and an air-conditioned hotel, the Villa Arqueologica Coba, provides shelter for about $35 double. The hotel has a swimming pool, a restaurant and a library of books on local history and archeology.

Coba was founded about 1,300 years ago near a pair of lakes. The hotel overlooks one of the lakes, and a 10-minute walk along the shore brings you to the ruins, or at least those that have so far been uncovered.

The entry fee, payable at a shack near the entrance, is about 35 cents.

Just inside and to the right of the entry gate, the Grupo Coba boasts a towering pyramid of jagged gray rocks. Climbing the narrow steps can be treacherous, and by the time you reach the top you’ll be thankful for having taken along a canteen of water.

From atop the pyramid, both lakes come into sharp focus. Surrounding them, stretching in every direction, is a sea of foliage in various shades of green and brown. To one side, another pyramid, Nohoch Mul, juts into the sky, high above the jungle and the narrow, muddy path that leads to its base.

The pyramid of Nohoch Mul towers more than 100 feet above the jungle floor--taller than the spectacular El Castillo at Chichen Itza and the Pyramid of the Magician at Uxmal.

In their present state, the 1,300-year-old pyramids of Coba don’t sport the precision of the perfect, four-sided structures at Chichen Itza. They seem, rather, to have been built into hills, with steps leading up only the front side.

The architecture of Coba, in fact, owes more to the style of the great Mayan cities of Tikal in Guatemala and Copan in Honduras than to the Toltec-influenced pyramids of nearby Chichen Itza that came along about 500 years later.

Nohoch Mul is a 15-minute jungle walk from Grupo Coba, and the view from its top is equally spectacular--tangled foliage stretches for as far as you can see, much of it hiding ruins and roadways still to be uncovered.

Many of the great Mayan cities were abandoned to the jungle about 1,000 years ago. No one is certain whether an epidemic, earthquake, overpopulation or a general breakdown of the social fabric caused the collapse of this great civilization.

Exploring Coba is tiring, the heat is blistering and the mosquitoes are merciless. After a good two hours of baking in this outdoor sauna, you’ll yearn for a dip in the hotel pool.

Leaving Coba, a 45-minute drive north will deliver you to the main Yucatan highway connecting Cancun and Merida. A left turn and an hour and a half more will take you to Chichen Itza.

A couple of days amid the unparalleled ruins, pyramid, ball court and observatory at Chichen Itza will satisfy any hunger for more Mayan mystery.

But even Chichen Itza’s magnificent sights will not erase the memory of those quieter, perhaps more elemental ruins at Coba. The jungle trails. The uncrowded pyramids.

And the butterflies.

The only hotel in Coba is the Club Med-owned Villa Arqueologica Coba, which has air-conditioned doubles for about $35 a night. The hotel also has a swimming pool, restaurant and bar. For reservations, write to Hoteles Villa Arqueologica Coba, Avenida Masaryk 183, Mexico City, Mexico DF 11570.

Some of the village residents offer primitive accommodations for much less.

Daily flights to Cancun from Los Angeles are available on Mexicana and Delta Airlines for about $350 round trip. Nissan Sentras, Volkswagen Bugs and Jeeps can be rented at the Cancun airport for about $40 a day. Larger cars are significantly more expensive.

For more information on travel to Mexico, contact the Mexican Government Tourism Office, 10100 Santa Monica Blvd., Suite 224, Los Angeles 90067, (213) 203-8191.