Earlier this year, a team of archaeologists announced the discovery of more than 61,000 long-lost Maya roads, fortresses, drainage canals, and buildings hidden beneath the dense green canopy of northern Guatemala’s tropical forest. The findings were the result of an airborne laser, or lidar, survey of 2,144 square kilometers of the Maya Biosphere Reserve. Now, archaeologists are starting to piece together what all the data actually says about Maya civilization. The picture emerging from beneath the jungle canopy is of a much more complex political and economic landscape than we previously realized.

“In February, you were really getting our first impressions as we began exploring the data, which was filmed by National Geographic. The Science article is about putting actual numbers behind those first impressions,” Ithaca College archaeologist Tom Garrison told Ars Technica.

A network of cities

Garrison, with archaeologists Marcello Canuto and Francisco Estrada-Belli of Tulane University and colleagues, used the lidar survey data to estimate that a Mayan population of between 7 million and 11 million people lived in the central Maya Lowlands (parts of the Yucatan, Guatemala, and Belize) in the bustling Late Classic Period between 650 and 800 CE. With 29 structures per square kilometer in the survey area, and evidence from earlier excavations to suggest how many of those structures might have been houses, the team calculated an average of 80 to 120 people per square kilometer. The overall number lines up well with estimates from previous studies, even though it was derived independently.

What archaeologists haven’t known for sure, because of the small-scale nature of most excavations so far, is how all those people clustered among the rolling karst and seasonal wetlands of northern Guatemala. With the large scale of the new survey, however, archaeologists can see a network of densely populated cities, connected by invisible bonds of trade and political affiliation that appear on the landscape as elevated roads between settlements.

That image supports the view that researchers have collectively leaned toward in recent years: the Maya were not a scattering of isolated, warring city-states living off slash-and-burn farms. They built urban centers supported by intensive farming of drained and irrigated lowland fields and carefully constructed upland terraces. The largest cities seem to be surrounded by smaller satellite cities and patches of rural territory, which Canuto and his colleagues say were probably connected politically and economically.

Maya breadbasket?

To support so many large cities, the Maya would have had to modify the landscape in more drastic ways than just clearing some trees; irrigating fields or draining wetlands would be essential. The fact that archaeologists have been estimating a population in that range for several years suggested that the Maya probably practiced more intensive farming, but archaeologists didn’t have enough data to say how widespread it was.

But in the lidar images, the ghostly outlines of channels reveal grids across the low seasonal wetlands, called bajos, where they would have served as a combination of irrigation channels and flood control. Most are one or two meters wide, between 20 and 50cm deep, and the longest stretch for a kilometer across the low-lying fields. In the uplands, lidar images revealed the long outlines of terraces and low field walls.

All told, about 17 percent of the survey area seems to have been farmland. But that varied from less than one percent in some blocks to as much as 70 percent around large cities like Tikal. Even with so much land under cultivation, and even with a lot of labor and infrastructure invested in getting the most out of the land, however, some of the larger Maya cities would have needed to rely on imported food to feed their people.

Canuto et al. 2018

Canuto et al. 2018

Canuto et al. 2018

Canuto et al. 2018

Trade and transport

“It is clear that some of the most densely settled cities, like Naachtun and Tikal, would not have been able to support their population with the land available to them,” Garrison told Ars Technica. “Other kingdoms, like Holmul, definitely had surplus. I suspect that yes, there was a certain degree of exchange of staple foods.” Further lidar surveys may help shed light on agricultural systems and which kingdoms controlled which chunks of farmland.

There's evidence of formal links between cities during earlier periods in Maya history in the form of elevated roads called causeways. These link many large cities with the smaller urban centers nearby; in a few cases, they link major cities to each other. These are broad roads, 10 or 20 meters wide, and the longest runs 22km through what was once farmland.

But most of the causeways that show up on the survey link cities that rose to prominence between 1000 BCE and 250 CE. Because archaeological evidence suggests that most Preclassic (1000 to 250 BCE) cities were abandoned by the Late Classic Period, Garrison says the causeways between them probably also fell into disuse. Sometimes that abandonment stood out in stark relief in the lidar images.

“In one interesting case from the lidar, agricultural terraces associated with the Classic-period kingdom of Xultun are constructed over a Preclassic causeway that had emanated out of the nearby site of San Bartolo,” he told Ars Technica. “In this case, the lidar is literally revealing the layering of settlement over time.”

Fortresses to withstand a siege

What the Maya were building during the Late Classic Period, however, were fortifications—and lots of them. Defensive systems of bridges, ditches, ramparts, and stone walls lie hidden beneath the foliage in greater numbers and at greater scale than Canuto and his colleagues expected, even though texts and archaeological evidence both portray the Maya as a militaristic, conflict-prone people.

“Their own writing gives sometimes vivid descriptions of war, in one case using hurricanes as a metaphor for the fury of battle, and in another describing its aftermath as the ‘piling of skulls and the pooling of blood,’” Garrison told Ars Technica. And archaeological work at Tikal and a few strategic points farther west has revealed some earthworks.

The defensive structures that showed up on the lidar survey, however, were much more common, and much more sophisticated, than most archaeologists had expected. And that suggests that Maya civilization must, at least at times, have engaged in warfare on an even larger scale than anyone thought. As evidence, Garrison points to a fortress revealed by the lidar survey, perched on the edge of an escarpment between Tikal and the nearby city kingdom of El Zotz.

“The citadel is protected by walls that are over 25 feet tall, and there is a large artificial reservoir that looks like an Olympic swimming pool. In other words, this place (named La Cuernavilla) was ready for a siege,” he told Ars Technica. “That is not really the type of conflict that we think about for the ancient Maya.” Garrison is about to begin a three-year excavation at La Cuernavilla.

Lasers in the air and boots on the ground

Many of the features that showed up in the lidar images are much harder to spot from the ground. Causeways, irrigation channels, and small defensive earthworks often leave a pretty subtle mark on the landscape after a few centuries, and their scale extends well beyond a person’s field of view in the dense jungle. “Even knowing a causeway is there and having lidar imagery loaded into my GPS, I still need to search carefully to be sure that I am in fact recognizing its edges,” Garrison told Ars Technica.

But there are still years’ worth of work to be done on the ground to confirm that what Canuto’s team saw in the digital images is actually there beneath the tropical forest canopy, a process archaeologists call ground-truthing. That process began before the survey even started; by choosing survey areas with known archaeological sites, the team could use those sites to help calibrate what they saw in the images. And more ground-truthing fieldwork started in 2017, shortly after the completion of the survey flights.

“In general, the lidar is extremely accurate, missing only the most subtle structures. There are occasional false identifications, but that seems to be restricted to certain types of terrain,” Garrison told Ars Technica.

Meanwhile, the survey has revealed some new sites for archaeology, such as Garrison’s upcoming work at La Cuernavilla, and provided a broader context for sites that have been excavated for years. And the broad view of the Maya landscape still leaves plenty of questions unanswered; there’s lots of detail to fill in on the big picture, and that detail may shape how archaeologists eventually interpret what they see in the lidar images.

And more large-scale views of the ancient landscape may be forthcoming, since the 2016 survey covered only portions of the Maya Biosphere Reserve. The Foundation for Maya Cultural and National Heritage, or Pacunam, has plans to cover most of the rest of the reserve in the coming years.

Science, 2018. DOI: 10.1126/science.aau0137 (About DOIs).