"Ay, Chihuahua!" yelled the startled young Mexican tourist as his foot slipped on a rock and he almost fell 70 feet into the Sacred Well of the Maya. "For being a dumb ox," he told his friend to whom he had been explaining the history of the Cenote Sagrado, "I almost became a sacrifice myself!" Hastily they moved away from the unguarded edge of the sheer limestone cliff-wall.

After an absence of nearly eight years, I had again walked about 300 yards along the dusty path cut centuries ago by the Maya from the Grand Plaza of the Castillo through dense vegetation to the sacrificial cenote or well.

The heat and humidity in July and August (as I knew from past experience) can be as unpleasant amid these massive stone ruins as on one of Washington's worst stagnant summer days, with the mercury rising well above 90 and the additional factor of a blazing tropical sun. Even though there's no air pollution.

Usually it's more comfortable to climb the unshaded pyramids in winter or early spring, when temperatures normally are about 10 degrees cooler and the rainy season has ended. But the sun this April morning, even in a partially cloudy sky, was brutal enough to parch the throat, turn cheeks blood-red, and drench the entire body in sweat.

In a few minutes, the forbidding walls of the yawning natural well loomed ahead, with the remains of a small temple-platform at one side. From there, in rituals supervised by the priesthood, drugged victims (many of them prisoners of war) and perhaps some volunteers eager for the "honor" of interceding with Chac, the powerful Maya god of rain, were pushed or jumped into the cenote.

Some did not die from the fall or drowning, and were later brought out - ostensibly qualified to give predictions about future rainfall after communing with the god.

The 124-mile journey by rental car from Cancun on the Mexican Caribbean to the ruins of Chichen Itza - clearly the number one side trip from the booming young resort city - proved to be a rather monotonous 2 1/2-hour drive. The good, two-lane hard-top road with only gentle curves and very few crossroads rolls through the mostly deserted contryside of Quintana Roo and then Yucatan, the two states most distant from the Mexican capital. On some stretches (in the absence of anything resembling traffic flow) I hit 120 km per hour - about 75 mph - following the example of the Mexican drivers who generally ignored the posted limit of 90 km.

Jungle-like growth (not lush rain forest) lines both sides of the well-maintained highway for mile after mile, except for short stretches where tiny Indian villages, pockets of poverty, are revealed. Here the Maya, many fullblooded, live much the same as their ancestors did, often worshipping some of the same gods, including Chac.

"Why did they believe their god lived in the Cenote?" The darkskinned, middle-aged Mexican guide posed the question to a family of foreign tourists who had hired him. These were the same English phrases he used almost in sing-song fashion day after day at the famed site.

"Because there are no surface streams or rivers in Yucatan." he answered himself, explaining that rain would seep through the porous limestone ground and be trapped below the surface by a hard layer of clay, thus forming underground pools and rivers. At Chichen Itza, the limestone cap had collapsed, forming a natural sinkhole.

Since the Maya wanderings were often dictated by their need for water, and since water was life, the guide continued, the Indians concluded "their rain god must be living at the bottom of the cenote." The tribe was called Itza, and the name Chichen Itza means simply "at the edge of the well of the Itza."

Again I found myself filled with a sense of wonder about the people who has build the still-impressive structures that surrounded me.

The Maya, whose origin remains unclear, were immersed in religion and scientific discovery at a time when Europe was in darkness. Their priests were so adept at astronomical observances that they devised a calendar as accurate as the one we use today, predicted eclipses, understood and used the concept of zero many centuries before the Arabs, and created a system of hieroglyphic writing that scholars are still trying to fully decipher.

Some experts believe that the Maya may descend from the mysterious "Olmec", a largely unknown civilization that inhabited the Gulf Coast of Mexico about the time of King David, more than 1,000 years B.C.

The most common theory holds that all the Indians of Mesoamerica descend from bands of wandering hunters who migrated from Asia via a land bridge that originally existed across the Bering Straight. Successive migrations occurred beginning at least 12,000 years ago, and perhaps as many as 40,000 years.

But some researchers maintain that there was also a migration from Southwest Asia across the Pacific Ocean, and that these members of an advanced civilization brought with them the knowledge responsible for the unusual advances in astronomy, mathematics, architecture and art made by the primitive Indians of this region.

There are other theories, some ridiculed by archeologists - such as the postulation of a common originating point from which was populated America, Europe, Africa and Asia - or the more scientifically buttressed theory of cosmic catastrophes best advanced by Immanuel Velikovsky in his "Worlds in Collision", which holds that successive world civilizations have been abruptly destroyed and suggests a plausible and fascinating genesis for the feathered serpent god. Another, more esoteric, view holds that superior beings from outer space visited this planet ages ago, imparting wisdom to a lost civilization that flourished long before the Egyptians, and that bits of their wisdom was handed down to the Egyptians and the Indians.

In any case, during the last portion of the Pre-Classic of Formative Maya Period, from perhaps 100 AD to 600 AD, the Roman Empire had disintegrated and the Huns swept across Europe from Asia. During the Classic Maya Period, from about the 7th to the 11th centuries, most of the structures at Uxmal (OOSH-mal) were erected.

Uxmal, which I had also visited the last time, and was not on my itinerary this trip. Located about 50 miles south of Merida, capital of Yucatan, it was a major city in the Puuc region (named for its style of architecture), but covers a much smaller area than Chichen Itza and can be seen in half a day. Chichen lies about 75 miles east of Merida.

In the Classic Period, the Maya occupied a territory that corresponds today to the Mexican state of Yucatan, half the states of Campeche, Tabasco and Quintana Roo; Guatemala; Belize (British Honduras) and the western part of Honduras. The inhabitants, who shared the same culture and religion and whose economic life revolved around the cultivation of corn, spoke different dialects and lived in autonomous provinces similar to city states rather than being part of a formal empire.

Uxmal's clean lines and spacious design, the beautiful record of stonemasonry, is more purely Maya than what can be found at Chichen. Absent is the fervid cult of blood, terror and war usually associated with other great Mexican archeological sites, such as mysterious Teotihuacan and Aztec centers of power.

Because sometimes between the 10th and the 11th centuries, the more war-like Toltec from Central Mexico imposed their will on the Maya of Chichen, who were thus forced to modify their religion.

"The new religion involved human sacrifices to a degree that the Maya had never known before," according to the Instituto Nacional de Antropologia e Historia. It also involved more aggressive militarism (recent discoveries indicate the Maya went on raiding expeditions from earliest times) to conquer other groups and obtain more sacrificial victims.

But I had to ask myself that if the Maya thus increased the emphasis on human sacrifices in their religion, was this admittedly savage practice any more revolting than the bloodletting embraced - or at least tacitly condoned - before and since by more "civilized" races and religions?

According to the sacred Maya book, the Chilam Balam, Chichen Itza was first occupied from about 495 to 692 AD, at which time the Itza left to live in another city for approximately 200 years. The tribe returned to Chichen from 987 to 1185, and around that time the cultural influence of the Toltecs was brought to bear on the Itza, with the foreigners eventually assuming political control.

This caused "profound changes" in the lives of the Itza, according to the Instituto Nacional, and plumed or feathered serpent god Quetzalcoatl (whom the Aztecs also worshipped in various forms) became the Maya god Kukulkan. From the 10th to the 13th centuries, Chichen shone as probably the most important sacred city in Yucatan. During that period were built many of the structures I was revisiting.

From the middle of the 13th century, less than 100 years before the Spanish Conquest of Mexico, Chichen began to lose its prime position, though offerings and sacrifices were still being presented at the Cenote Sagrado from the entire Maya region. Finally the city was abandoned to the jungle. In 1923, the Mexican government and the Carnegie Institution of Washington began explorations and restorations at Chichen, and the work continued for 20 years.

And so I gingerly approached the edge of the well, looked down, and remembered that divers and succeeded in bringing up bones, jewelry and other artifacts from the depths - though the romantic story of bejewelled young virgins being sacrificed has generally been discredited.

Then, swatting at persistent mosquitoes, I walked back to the broad, sunbaked plaza and began to climb what is perhaps the most imposing monument on the site: the Temple of Kukulkan, better known as El Castillo.

I reached the top in better shape than some gasping two-pack-a-day smokers. El Castillo, whose plumbed serpent columns at the doorway to the temple reveal the Toltec influence, apparently also had some relation to sun worship. It stands 78 feet above ground, but seems to be a least twice as tall because the steep, narrow steps exact much effort. Each of its four sides has 91 steps. Addition of the temple platform brings the total to 365, the number of days in a year, while each of the pyramid's sides is faced with 52 stone slabs, the number equaling the Mayan cycle of years. I walked through the chambers, visualizing sacrifices being made to Kukulkan atop the pyramid in much the same way sacrifices were offered at the Pyramid to the Sun in ancient Teotihuacan and by the blood-stained Aztec priests in Tenochtitlan (now Mexico City). The victim's chest would be slashed open swiftly and the throbbing heart would be presented as a gift to the gods.

I looked down the sides and watched my fellow tourists of all ages laboriously climbing up or down, some using the chain that runs down one face. Most had arrived on bus tours (about $23 per person, round trip, including the 75-cent admission to the archeological zone, guide service and lunch). Buses leave around 8:30 a.m. and return to the Cancun hotels in late afternoon.

To give me more flexibility than a bus tour, I had chosen to rent a car from Avis (Hertz has no vehicles in Cancun with automatic shift, and neither company offers power steering, a decided nuisance in the tropics).I had returned my first car after one day because I didn't like the way it was running and because it had 69,000 miles on it. The Dodge Dart had made it to Chichen many times before, I imagined, but I wanted a newer car.

The accommodating Avis personnel at the Hotel Presidente, some of Maya descent, displayed typical Mexican warmth, I remembered. They found me another Dart with only 15,000 miles. Afterward, when I indicated I would be writing a story and would have to mention that some of their cars were a bit heavy on mileage, the manager smiled brightly, gave a characteristic Mexican shrug, and reminded me gently:

"Si, pero son de buen corazon" (yes, but they have a good heart).

He was right. I had had no trouble on the road. But the sense of isolation is so pervasive - I don't know what help could be expected from one of the blackward villages - that a mechanical breakdown might mean hours of waiting for some vehicle to stop. And I had not seen a single member of Mexico's vaunted highway tourist auxiliary on the main road from Cancun to Merida that directly bisects the ruins of Chichen.

But now the view from El Castillo's summit held my attention. The panorama of the ruins was splendid. They are spread over a number of square miles. There are more than two dozen major structures, but scores of other edifices are still buried, some pinpointed by grassy mounds, since no funds exist for excavation and restoration. Mexico has more urgent human needs to meet now.

Across the plaza was the striking Temple of the Warriors, whose dozens of columns bearing carved figures of warriors mark it as clearly Toltec. The column, according to Hans Helfritz in "Mayan Cities of the Gods," was "the greatest idea to enrich the skills of the Maya builders." Until the column was introduced as a way of supporting roofs, the Maya had been limited to narrow corridors and false arches.

Later, there would be time to visit the Warriors, and also the Market Place, the Ball Court, Temple of the Jaguars, the Steam Bath, and other structures. But first, before walking to the nearby Mayaland Hotel for lunch, I retraced my steps down on side of El Castillo and entered a small door below.

During reconstruction of the pyramid, archeologists discovered that a smaller pyramid and temple had been enclosed by the builders of El Castillo. For only a few hours each day, visitors can climb single file up the narrow steep stairway in dim light to the inner chamber of the hidden temple.

After what seems an interminable climb with little oxygen, but which actually takes only a matter of minutes, you come face to face with a stone Chac-Mool idol (a figure half lying, half sitting, which combines the rain god and the jaguar). Next to it is a stone jaguar, painted red and inlaid with jade, believed to have been a throne for the high priest. The figures have not been disturbed for centuries, and there is an eerie feeling that the priests will return at any moment.

After squeezing back down the same stairway past a long line from the tour buses and out into the fresh air again (no advisable for heart patients or claustrophobics), one discovers that many tourists have turned back because of the crush. Certainly Chichen Itza will be receiving thousands more visitors due to the opening and growth of Cancun, so perhaps the Mexican authorities should consider lengthening the hours when EI Castillo's inner temple is open - and limiting the number of bodies permitted inside at any one time.

Back at the Mayaland, where I had parked the car, I got a table in the dining room before the tours crowded in and enjoyed one of my most delicious meals in Yucatan featuring Pollo Pibil, chicken wrapped in banana leaves and baked. The other equally excellent entree on the fixed menu was Huachinango a la Veracruzana (fresh Red Snapper in a tasty tomato sauce). The lunch, unfortunately limited in scope by the need to rapidly feed the tours, also includes a small fresh fruit cup or glass of tomato juice, a tiny nondescript "salad" (which most gringos probably avoid anyway as a health precaution), bread and dessert (custard or pastry). The price is 100 pesos ($4.75), plus beverage if other than tea or coffee, and tip. Live regional guitar music was nearly drowned out by the chatter.

My only real quarrel with the dining room (other than the fact that I did't get enough chicken) was that, despite screening, the place was alive with flies that had to be fought throughout the meal. Surely the Mayaland, a beautiful colonial-type structure with high ceilings, a lovely garden and a willing staff, can do something about that.

Later, before again visiting the ruins prior to driving back to Cancun, I stood alone in the main foyer and looked out the hotel's entrance that frames the ruins of the Maya observatory, El Caracol, not far away. (Because of the hotel's proximity to the ruins, and the size of the area the best way to tour them is to spend a night at Mayaland, which allows nearly two days to see the structures without rushing).

At El Caracol the Maya priests viewed the heavens. Repository of the secrets of the planets, the calendar, and other knowledge vital to an agrarian culture dependent upon accurate planning for the rainy season, the priests were an all-powerful ruling class. But I recalled that eventually, as scholars now believe, the laboring class rebelled.

They revolted and killed many of the priests, but not in protest of cruelty - after all, Western man, that inveterate finger pointer, has rated most Indians as "savages" - but rather because they got tired of carrying such a heavy work burden. And with the loss of their leaders, and the demise of religion that was the stimulus for science and construction, Chichen Itza declined.