Getting Physical Inside a Pyramid

Getting Physical Inside a Pyramid By MARION LLOYD FEBRUARY 18, 2005 Scientists hope subatomic particles will help unlock the mysteries of an ancient Mexican monument Equipped with hard hats and rubber flashlights, a team of Mexican physicists begins its slow descent into the bowels of the world's third largest pyramid. The scientists wind their way through the cramped, water-soaked passageways of an ancient tunnel beneath the Pyramid of the Sun at Teotihuacán, retracing the steps of the mysterious people who settled here more than 2,000 years ago. At its peak between AD 300 and 600, Teotihuacán was a vibrant city with hundreds of thousands of residents, making it perhaps the world's largest metropolis of its day. It dominated trade routes from central Mexico to Honduras, and its people erected enduring monuments, unrivaled in the Americas, that still loom over the landscape 30 miles northeast of Mexico City. But Teotihuacán (pronounced tay-oh-tee-wah-CON) has baffled archaeologists for more than 150 years. Excavations across seven square miles have turned up precious few clues about who governed this vast city and how it managed to thrive for centuries. Now a group of physicists armed with a powerful particle detector hopes to shed some light by exposing the interior of the city's largest pyramid -- without moving any of its stones. From their makeshift laboratory in the underground tunnel, the scientists are working to take a giant CAT scan of the 216-foot-high Pyramid of the Sun, which is surpassed in size only by the Egyptian pyramids. The goal is to find out what, if anything, is hidden within the near-impenetrable mass of rock and earth. "This shows that science can have a social impact, that it can resolve mysteries," says Arnulfo Martínez, one of the physicists from the National Autonomous University of Mexico, or UNAM, who are conducting the experiment. He leads the way along the dank passageway, its walls coated in copper-colored slime. Excess moisture caused a jury-rigged lighting system to short out weeks before, and the tunnel is pitch-black. The passageway's low roof -- under five feet in areas -- has also caused logistical headaches, forcing the scientists to lug in their

detection equipment one piece at a time, before assembling it in a 10-foot-high cave at the end. "Still, we're very grateful for the tunnel," says Mr. Martínez, noting that without it, the experiment would not have been possible. The equipment allows the researchers to scan for subatomic particles, called muons, which are formed when cosmic rays from distant stars collide with the earth's atmosphere. The muons rain down uniformly and are absorbed when they interact with matter, such as the rock inside the pyramid. By detecting variations in muons passing through different parts of the pyramid, the physicists hope to locate hidden chambers or collapsed tombs. The discovery would then allow archaeologists to excavate the findings for more clues about the city's ancient inhabitants. "I hope they do find something, because that would increase our ability to recreate the story of Teotihuacán," says Rubén Cabrera, a Mexican archaeologist who has spent the past seven years excavating inside the neighboring Pyramid of the Moon. The Álvarez Test The project is modeled on an experiment carried out in the late 1960s by Luis W. Álvarez, a Nobel Prize-winning physicist at the University of California at Berkeley. Mr. Álvarez set up a system of muon detectors in a tunnel at the base of the Khafre Pyramid, in Egypt, to determine the possible existence of hidden tombs. After a year of mapping, he concluded that the monument was completely solid. The "Álvarez test" sparked the interest of Arturo Menchaca, director of UNAM's Institute of Physics, who is in charge of the mapping project at Teotihuacán. While conducting postdoctoral studies at Berkeley in the mid-1970s, he asked Mr. Álvarez whether the test could be applied to the Pyramid of the Sun. But the response was discouraging. "He told me I would need access to an underground tunnel," says Mr. Menchaca, a nuclear physicist with an interest in cosmic rays. At the time, neither man knew that such a tunnel had just been found beneath the pyramid. (The entrance to the passageway was discovered in the early 1970s thanks to a scavenging dog.) In 1978 an archaeologist at UNAM, Linda Manzanilla, also asked Mr. Álvarez if the muon detectors could be used at Teotihuacán. But it wasn't until 2001 that she and Mr. Menchaca discovered their shared interest and teamed up to carry out the experiment.

The $500,000 cost, a huge sum by Mexican research standards, is being paid by UNAM and Mexico's National Council for Science and Technology, the main government agency that awards scientific grants. Most of the money has gone toward the detection equipment and monitors, with parts that had to be imported from the United States and Europe. The scientists then built the systems virtually from scratch with help from UNAM engineers. "No one had ever tried to build such large detectors in Mexico," says Mr. Martínez. He describes the nerve-racking process of soldering microscopic wires onto four-foot copper panels. The technology is most commonly used by mining engineers to detect hidden tunnels. But if the project at Teotihuacán proves successful, researchers could apply it on any archaeological ruin that has a hollow space beneath it. "From a physics perspective, the project isn't that demanding," says Mr. Martínez, referring to the relatively simple principle behind the experiment. But, he says, "the mere fact of building the equipment and then making it work inside the pyramid -- that's interesting." Pile of Rubble The final system will consist of three pairs of copper and acrylic chambers, designed to capture muons racing by at nearly the speed of light. As the particles zoom through a gas inside the chambers, they slam into atoms with so much energy that they knock electrons free, producing an electric pulse. By measuring those pulses, scientists can count the number of muons going through different sections of the pyramid. Most of the equipment is still being built in the institute's lab, although the scientists have been conducting preliminary tests with a prototype installed inside the pyramid. Unlike the Egyptian pyramid Mr. Álvarez studied, which is made of solid granite blocks, the Pyramid of the Sun is an irregularly shaped mass of volcanic rubble and earth. So the scientists had to go the added trouble of calculating the average density by taking samples from inside the pyramid. They then calibrated the detectors to take into account the monument's lopsided surface. Once the system is ready to go, probably by April, the physicists will install it inside a prefabricated metal shed in the largest of several caves linked by the tunnel. (The casing is airtight to protect the equipment against moisture, as humidity levels inside the tunnel hover around 95 percent.) The mapping process should then take about a year, the time the scientists estimate it will take to get a

clear picture of the pyramid. The level of precision will allow them to detect any spaces larger than a medium-size television, which they believe is sufficient to identify a possible tomb. "We could spend more time and find even smaller spaces, but it wouldn't be practical," says Mr. Martínez, as he tries to locate where the short is in the tunnel's lighting system. Beginning at the base of the pyramid's steps, the passage runs for several hundred feet to the center of the monument, dead-ending at a 10-foot-high cavern surrounded by four smaller caves. Hulking stone fragments of an ancient aqueduct -- possibly used to carry water out of the tunnel -- lie scattered along the passageway. Many scholars believe the city's early inhabitants created the tunnel, taking advantage of underground caves, and then built the pyramid on top to venerate the site. "They say this was a sacred place for the teotihuacanos," says Mr. Martínez, pointing out 2,000-year-old finger marks in the original mortar shoring up the tunnel. However, he confesses that he is primarily interested in the practical applications of the tunnel, which provides an ideal base from which to map out the 34 million cubic feet of material overhead. It is a solution born of necessity. Beginning in the 1920s, archaeologists dug three tunnels halfway through the pyramid at different heights, looking for signs of a construction system -- and artifacts. But they turned up only pebbles and dirt. "When they saw it was a pyramid of solid earth, they lost heart," says Mr. Menchaca, the project director. "There are no clues that tell you where to keep digging." In contrast, the smaller Pyramid of the Moon was built onion-style, with increasingly larger layers added on. During excavations that concluded last fall, archaeologists used the internal structure to find six sacrificial burial sites, which included the skeletons of more than a dozen sacrifice victims, ornate jade masks, and obsidian serpents. Those finds have triggered hopes that even more spectacular objects lie hidden within the Pyramid of the Sun. Ideally, researchers wish to find the tomb of a ruler -- or co-rulers -- that would resolve nagging questions about the way the city was governed. Unlike the ancient Maya, who built elaborate tombs for their rulers and then recorded their names and dates in the artwork, the inhabitants of Teotihuacán left no references to their leaders anywhere -- at least none that archaeologists have found.

A Reticent People? The mystery is particularly intriguing, considering the city's enormous influence in Mesoamerica from about AD 1 until its collapse around AD 650. At its height, after AD 300, Teotihuacán was home to as many as 250,000 people and controlled trade across a wide region. The absence of royal records matches the inhabitants' reticence about life in general. They left only a few written symbols, which researchers have been unable to decipher. As a result, many conclusions are based on what Teotihuacán was not, rather than on what it was. For example, the city is unusual in its lack of images glorifying human sacrifice, which are so prevalent in the Maya and Aztec cultures. While archaeologists have unearthed scores of decapitated skeletons at the city, the practice does not appear to have been as central to the culture as in most of Mesoamerica. Rather than showing bloody scenes of human sacrifices, the murals at Teotihuacán instead depict scenes of wildlife and benevolent-looking priests. "It was a very different type of state," says Ms. Manzanilla, the archaeologist who is collaborating on the muon project. After 30 years of excavating at Teotihuacán, she says she is far from cracking the mystery. But she has developed a hypothesis regarding its government system that she believes could shed some light on the enigma. Rather than the single-ruler model dominant in Mesoamerica, Teotihuacán was governed through a "corporate system," in which multiple leaders -- most likely four -- shared power, she proposes. "This is the biggest city in Mesoamerica, and we don't have any names. You can't tell me that if they had one great leader, we wouldn't see him represented," she says, as she supervises students excavating several human skeletons in an ancient "neighborhood center" nearby. (She suggests that the complex, called Teopancazco, could have been the administrative offices of one of the co-rulers. During excavations last fall, she discovered 30 decapitated skulls buried at the site, which she argues shows that the buildings served an important purpose.) Such a governmental system could explain the apparent political stability of the city, as compared with that of the Maya, who frequently suffered military coups, she says.

"The only way to ensure the survival of a city of 150,000 people was if they heard all the voices, of all the groups," she says, as she leads the way to another complex, known as Xalla, which she is excavating near the Pyramid of the Sun. Unlike most of the smaller buildings at Teotihuacán, which are arranged in groups of three, Xalla comprises four nearly identical buildings around a central monument. Ms. Manzanilla suggests that the complex may have been the city's administrative palace, and that the four buildings symbolize the equality of the four rulers. She notes that the number four is an important symbol in the art and urban design of Teotihuacán, such as in the ubiquitous paintings of four-petaled flowers and the four lobelike caves beneath the Pyramid of the Sun. Another proponent of the "corporate governance" theory is Esther Pasztory, an art historian specializing in Mesoamerican cultures at Columbia University. In her book Teotihuacán: An Experiment in Living (University of Oklahoma Press, 1997), Ms. Pasztory argues that Teotihuacán was unique in the region in developing a government based on a system of checks and balances. She bases her theory on evidence of a more egalitarian culture in the city, which extends to its urban design. "Most [Mesoamerican groups] had palaces for royalty, and thatched huts for ordinary people," she says. Instead, Teotihuacán had an elaborate system of stone housing complexes clustered alongside the pyramids and stretching for miles across the surrounding valley. "This idea of upper middle-class housing, multifamilies ... it's unique in the New World," she says. "It means there was enormous work put into the housing of the ordinary population." She goes further to argue that the people of Teotihuacán deliberately moved away from their Mesoamerican roots, creating their own version of utopia. The culture might have more closely resembled that of the Incas in Peru, who were organized along a complex system of local governments, rather than the Aztecs, who were ruled by autocratic priest-kings. She cites the peaceful imagery of the Teotihuacán murals -- so different from the rest of Mesoamerican art -- and the apparently highly organized social structure. The city was the first example of complex urban planning in the region, with streets laid out in a grid and residential complexes that apparently housed roughly equal numbers of people. "They had a vision, a social and religious vision, and it lasted very successfully for 600 years," Ms. Pasztory says. "Then it collapsed, and no one ever tried that again."

It is a controversial view. Most archaeologists working at Teotihuacán believe there is not enough evidence to conclude that the city departed from the dominant one-ruler model. "Why should Teotihuacán separate itself from the tradition?" says Mr. Cabrera, the veteran archaeologist who co-directed excavations in the Pyramid of the Moon. "From the time of the Olmecs [around 2000 BC], there was always one leader." However, he admits to having doubts. "There is something very strange about the culture of Teotihuacán," he says, seated in front of a mountain of potsherds and other artifacts at his office inside the ancient city. "They don't show who the big men were. Why? Why didn't they give them the respect they were due?" Slim Chance He and the other scholars are hopeful that the muon experiment will help shed new light on the debate. "What we're looking for is some kind of proof to reinforce one of these theories," he says. It may not be easy. Many scholars, including those involved in the project, say there is only a slim chance of uncovering burial chambers inside the Pyramid of the Sun. For one, no sealed tombs have been found anywhere at Teotihuacán. Nor is there any evidence that the city's inhabitants buried their dead -- as opposed to cremating them -- since the only skeletons found so far appear to have been part of sacrificial offerings. (Ms. Manzanilla believes the Pyramid of the Sun was a giant monument to fertility, and that any skeletons found inside would most likely have been part of sacrificial offerings.) Even if they had sought to make an exception by burying their rulers inside the pyramid, there were structural impediments. "It wouldn't have been possible. They didn't have the technology to build stone chambers, much less within a pyramid of this size," says Alejandro Villalobos, a UNAM architecture professor who specializes in pre-Columbian cultures. If there were any such tombs, he says, they would have collapsed under the pyramid's weight. A more likely scenario is that the physicists could find denser areas containing more sacrificial burials, like those of the Pyramid of the Moon. The technique involved constructing outer walls from stone or adobe, and then laying the remains in a kind of open box. The tomb was then filled in with rubble, creating a denser space than the rest of the pyramid.

Even if the physicists come up empty-handed, the experiment will have served a purpose in sparing archaeologists another 150 years of fruitless searching. "I don't think we'll find anything," says Mr. Martínez, as he leads the way back out of the tunnel and up the rusting metal staircase. "But we can demonstrate that there isn't anything there. And that's a result."

http://chronicle.com Section: Research & Publishing Volume 51, Issue 24, Page A14