A Mayan engineering legacy - Coba.

By 700 A.D., the Mayan city of Coba covered 30 square miles, had an estimated population of 50,000, a 120-foot-high pyramid, and was at the hub of a roadway system connecting it with the other powerful cities of the Yucatan peninsula. Two centuries later, it was left almost abandoned.

Today, scientists are attempting to solve the mystery behind this city that was one of the wonders of Mesoamerica. Approximately 80 miles south of the present-day resort city of Cancun on Mexico's Yucatan peninsula, archaeologists are digging through layers of dense jungle. From the top of the Nohoch Mul, the 120-foot-high pyramid that is the capstone of the city's engineering legacy, they can see some 6500 mounds spread throughout the jungle. Under each mound is a structure that served a purpose in the daily lives of Coba's residents.

Although these ruins were first discovered in 1886, exploration did not begin until the late 1920s and early 1930s. But it was not until 1972 that an organized effort was made by the National Geographic Society in conjunction with the Mexican Institute of Anthropology and History to uncover the city. A program funded by the Mexican government continues to explore and study Coba, but progress has been slow. Dense vegetation covers the ruins, creating the appearance of mounds in an otherwise flat jungle. Although less than 10 percent of the mounds have been excavated, archaeologists are convinced that Coba was one of the largest Mayan settlements on the Yucatan peninsula.

Today's experts have considered whether the engineers of Coba devised an advanced technology to design structures that could stand for more than 1000 years. The evidence of recent digs, corroborated by progress scholars are making in deciphering Mayan glyphs, indicates otherwise. Archeologists believe that the Mayans used neither the wheel nor the pulley and that no beasts of burden lived in the area at the time. Instead, the engineers of Coba used plentiful native labor to construct the very advanced urban center. Coba's pyramids, temples, platforms, and civil works were made of precisely cut limestone blocks from nearby quarries, which were transported to their respective sites exclusively by human labor. The infrastructure could support the population of 75,000.

"In their engineering, architecture, and construction, the Maya simply substituted labor for technology," said Brian Dillon, field archaeologist and professor of archaeology at the University of California at Los Angeles and an expert on the cultures of Mesoamerica. "What they drew upon was the patience and deep-seated work ethic of the Mayan people. "

Dillon believes that Mayan engineers experimented with plumbing, air ducts, and sewage disposal. At Palenque, another major ruin site about 300 miles southwest of Coba in Chiapas State, channels for running water have been found beneath the floors of houses and other buildings.

The Maya of Coba were known to have developed a numbering system and an alphabet. In addition, the residents of the region practiced a complex religion and were part of a sophisticated regional city-state network.

Bridge in Time

On a time line of engineering accomplishments through the ages, chroniclers place the engineers of the three most productive Mayan periods-Late Classic (600-800 A.D.), Post Classic (800-1000 A.D.), and Terminal Classic (1000-1200 A.D,)-between the engineers of Rome's monumental civil works and the cathedral builders of the Middle Ages. However, the construction techniques of the Romans, Greeks, Egyptians, Etruscans, and later European societies benefited from some form of technology transfer and cross-cultural fertilization of ideas. The Maya had no such outside influences. They independently conceived and erected urban centers.

Other previously excavated centers of Mayan civilization, such as Chichen Itza, Monte Alban, Kabah, Palenque, and Tikal, show how the structures of these city-states reflect their citizens' particular religion, culture, and stylistic differences.

The importance of Coba as a trading and transportation hub is evidenced by a refined system of hundreds of miles of roads or sacbeob that crisscross the entire peninsula. Visitors today can walk along the same stone paths used by the Mayan runners. Travel then was on foot, though the ruling class and priests occasionally used boats since there were no animals for either transportation or construction. Some excavated structures along the sacbeob have been classified as toll booths.

What caused the residents of Coba to walk away from their city even though it was not overrun by the Spanish? According to archaeologist Christine Woods of the University of Southern California at Long Beach, the most accepted theory is that a combination of encroachment by more warlike rivals like the Itza from central Mexico and failing agricultural productivity forced them out.

"The Maya knew little if anything about crop rotation and the jungle in that part of the world is a scrub forest, not a true rain or wet forest: there is only about eight inches of soil resting on a solid limestone base," said Woods. After several hundred years of planting the same type of crop, primarily corn, the once-rich soil gave out because it is so shallow. The drop in the food supply also might have been one of the reasons for feuds between the lower and elite classes," she said.

To deal with the burden of feeding large populations like Coba's, rulers commissioned engineering projects like canals, reservoirs, and the terraced fields. Dillon believes that Mayan engineers had some success reclaiming lakebeds and that fish farming was practiced.

Golden Age

After more than 300 years of relative prosperity and power, the urban infrastructure of the city broke down and the residents of Coba and other Classic Mayan cities dispersed into small villages and settlements. But at their peak, the Mayan engineers and projects were singularly remarkable.

The Maya have lived for approximately 4500 years in an area roughly half the size of Texas that includes the Mexican states of Yucatan, Campeche, Quintana Roo, and parts of Chiapas and Tabasco. It also covers all of Guatemala and Belize, western Honduras, and El Salvador. Coba and Tulum, a smaller city on a spectacular Caribbean site located approximately 37 miles cast of Coba, are in Quintana Roo.

Coba means "water stirred by the wind." This probably refers to the five shallow freshwater lakes-Coba, Macanxoc, Sacalpuc, Yax, and Xhanha-that attracted settlers to the area. Although the area is thought to have been inhabited since the pre-Classic period (approximately 200 B.C.), the earliest known monument is dated 623 A.D.

Today, four of five main clusters of structures are open to visitors. The sites are up to a mile apart by footpath through the jungle. Unlike other more fully excavated sites in the Yucatan, which are compacted into parklike arrangements, Cobs is spread out, creating fascinating feelings of discovery as you emerge from the jungle upon individual stelae (carved stone slabs) or pyramids.

The Coba Group. This is the primary group of buildings to the right of the entrance. Features include a 105-step pyramid, called the Church; a room with a corbel arch; and a ball court similar to those found in many pre-Columbian cities. Here, a game played with a rubber ball (rubber was first fabricated in Mesoamerica) was sometimes used to settle political disputes between city-states. The gracefully tapering corbel arch, seen throughout ancient Mayan cities, needed no keystone, unlike other structures of the same era that would have collapsed without one.

Las Pinturas (The Pictures) Group. Most limestone structures in ancient Coba were covered with stucco and then painted with brilliantly colored figures of Mayan mythology. On the walls at the top of the principal temple, it is still possible to see interpretive Mayan artistry, executed in shades of yellow, red, and black, as well as intact hieroglyphics.

The Chumuc Mul Group. This group is largely unexcavated with the exception of a pyramid named Estuco because of its stucco covering.

The Macanxoc Group. Many carvings are discernible on the stelae in this group, including a woman ruler standing on captives and flanked by kneeling figures. Stelae had several purposes in Mayan culture: they were placed in front of prominent structures and contained information about the building or important people of the era; they marked periods of time, usually in 20- and 50-year intervals; they were placed along sacbeob; and they were ceremonial.

The Nohoch Mul Group. The towering 120-foot Nohuch Mul pyramid does indeed seem to rise up from the jungle floor. The back side of the structure is still covered with jungle growth. Another nearby pyramid has a now-empty tomb.

The base of Nohoch Mul, the second tallest pyramid in the peninsula, measures 180 by 198 feet. The tallest pyramid, at Knich Kamo in Izamal, Yucatan State, is virtually inaccessible to the average traveler. Nohoch Mul's limestone blocks are uniform in size throughout. They measure approximately 15 by 6 by 6 inches. For the most part, little or no mortar was used between the blocks.

The steps on Nohoch Mul get so narrow that ascent can be completed only by stepping sideways. Archaeologists surmise that Nohoch Mul and other major Mayan pyramids were widened at the base and made taller at the top in phases during the reigns of various rulers.

At the top of the pyramid is a temple built in the style seen at nearby Tulum. The single-vaulted chamber has four openings that naturally air-condition the space. Neither the stonecutting nor the overall workmanship of the temple is as precise as that of the pyramid; estimates place the construction of the temple at about 1000 A.D. It is believed that the temple honors the mysterious figure found at Tulum, the Descending God, or god of the setting sun.

Coba's Sacbe System

Evidence of Coba's importance is its sacbe, the longest known system of roads in the Mayan world. Since the Maya had no wheeled vehicles or draft animals, Dillon believes that sacbe were "tangible expressions of formal political relationships between cities."

One route is over 60 miles long and extends in a straight line to Yaxuna near Chichen Itza at the center of the peninsula. It was a major causeway and is thought to have been extremely important for utilitarian and military purposes as well as pageantry. More than 50 individual sacbeob have been found, all radiating from the center of the city and dividing it into four sections.

All sacbeob are straight and well constructed, often passing through swamps, lakes, and other difficult terrain. Constructed of inner and outer retaining walls with a core filled with stone, these roadways were solidified with lime and then plastered to provide a smooth surface. The width varied from approximately 9 feet to 60 feet where there were plazas; the height ranged from one to three feet. One sacbeob rose 21 feet above ground level.

Ramps were sometimes used at large intersections or at important civic complexes. Other ramps accommodated irregularities in the terrain. In addition, sacbeob in wet areas had culverts, while others had defensive walls.

Archaeologists found a limestone cylinder weighing five tons along the Yaxuna sacbe. They identified it as an ancient road roller. Other structures for water resource control, including dikes, check dams and wells, have also been discovered.

Platforms of varying size and complexity provided the base of both public buildings and residential structures. Researchers hypothesize that the buildings themselves were pole-and-thatch structures over which plaster was applied. The limestone substructures gave stature to the buildings they supported and kept the occupants dry during the rainy season.

The Mayan engineers of Coba and Tulum devised a form of faux columns to give a feeling of grandeur to entranceways and define other spatial relationships. Rather than a monolithic carved piece of stone, these columns were formed of rounded chunks of stone placed on top of each other, secured by mortar, and then plastered over for visual unity.

The full breadth and scope of the accomplishments of the culture of Coba, which reached its zenith about a thousand years ago, will take decades to unfold as the tangle of jungle above the hundreds of mounds is peeled back and the structures beneath are studied, analyzed, and recorded. They were built without CAD/CAM, hydraulic lifts, cranes, or even the wheel. Dillon observed that what the Maya did do well was organize construction crews of corvee, or unpaid labor.

"The rulers allocated a certain number of days per year that every head of household or able-bodied person had to contribute to work on construction of buildings and public works," said Dillon. "Enforcing this rule required a great deal of political sophistication; if everyone did not pull his weight, the job did not get done. Some Mayan communities still organize public works in this manner," he said.

ACID RAINS ON COBA

Mayan cities were once ablaze in color. Today, Mayan monuments and sculptures face ongoing destruction at an alarming pace, according to Merle Greene Robertson, head of the Pre-Colombian Art Research Institute in San Francisco. In a study funded by the National Geographic Society, Robertson concluded that while the naturally humid climate also contributes to the destruction of painted surfaces in Coba and other Mesoamerican sites, increased levels of acidic precipitation account for most of their deterioration.

Acid rain in the Yucatan peninsula is the result of oxides of nitrogen and sulfur dioxide emitted from smokestacks in oil fields and uncapped oil wells near Coatzacoalcos and Cuidad del Carmen on the Gulf of Mexico, said Robertson. The emissions are carried into the upper atmosphere and acted upon by sunlight and rain. They fall on the monuments of Coba and nearby Tulum, approximately 350 miles from the oil-producing areas, in the form of sulfur dioxide and nitric acid.

The Maya of Coba regularly repainted their buildings. Fifteen layers were found on the outer face of the inset lintel spanning the doorway at the Temple of the Paintings, a small temple-shrine structure in the Las Pinturas group. The colors are in surprisingly good condition due primarily to protection afforded by the roof overhang. The interior walls, however, are in a "devastated condition," said Robertson. "Black scab, bacteria, and mold covers everything."

Many of the murals in Tulum, which was excavated before Coba, have been nearly destroyed by the soot and dirt emitted from the exhausts of the many tour buses that park directly in front of th to the ruin site. The buses idle their engines to keep their air-conditioning running. "This is clearly manmade destruction and could have been prevented," she said.

Another factor in the destruction of buildings and monuments is dry deposition, the deposit of debris that accumulates in crevices and ledges. Dry deposition is carried by the wind and deposited as pollutants. When rain comes in contact with the built-up debris that includes sulfur dioxide and nitric acid the mixture acts the same way as acid rain, by eating into the stone surface.

The structures of the Maya and other Mesoamerican groups would have deteriorated faster had they not been painted, according to Robertson. Her investigation showed that bare limestone is attacked much more rapidly than a painted surface. Where a broken surface is next to a painted surface, it is the limestone portion that first becomes covered with black scab and dry deposition. These deposits encroach upon the painted area and destroy it.

To document the extent of the problem, Robertson has recommended that the Mexican government as well as those of Guatemala, Belize, Honduras, and Cuba, conduct a formal study on the effect of acidic precipitation on the tropical environments of Mesoamerica.