 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. Classic Maya civilization in detail 
 
 Lidar (a type of airborne laser scanning) provides a powerful technique for three-dimensional mapping of topographic features. 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. This number is consistent with previous calculations, the researchers noted in the study, which was published online Friday (Sept. 28) in the journal Science. Canuto et al. These link many large cities with the smaller urban centers nearby; in a few cases, they link major cities to each other. [See Photos from the Maya Lidar survey] 
 
 "Since lidar technology is able to pierce through thick forest canopy and map features on the Earth's surface, it can be used to produce ground maps that enable us to identify human-made features on the ground, such as walls, roads or buildings," Marcello Canuto, director of the Middle American Research Institute at Tulane University in New Orleans, said in a statement. Science, this issue p. eaau0137; see also p. 1313
– Maya civilization was no mere collection of city states using slash-and-burn farming—that we learned earlier this year. Now archaeologists are looking deeper into an airborne survey that revealed a formidable civilization double the size of medieval England at its peak 1,200 years ago, Ars Technica reports. The survey, which used LIDAR (Light Detection And Ranging) to peek beneath the jungle foliage, revealed some 61,000 buildings, draining canals, fortresses, and roads across roughly 828 square miles. What archaeologists are saying about those details in Science: Population: About 7 million to 11 million people populated the central Maya Lowlands—an area including some of Belize, Guatemala, and Yucatan—between 650 and 800 CE, the Late Classic Period. How did researchers crunch that number? By counting the structures per square mile and estimating how many were houses. Big cities like Tikal likely had hundreds of people per square mile, per Discovery. Farming: A huge agricultural effort was needed to keep all those mouths fed, LiveScience notes. Imagine a complex grid of channels providing flood control and irrigation, with grids up to six feet wide and 20 inches deep, some stretching over half a mile. Still, densely populated cities like Naachtun and Tikal had to import food from other Maya kingdoms to survive. Causeways: In earlier Maya times, from 1000 to 250 BCE, cities were linked by elevated roads or causeways up to 65 feet wide and up to 13 miles in length—but they fell into disuse when so-called Preclassic cities were abandoned. Yet their faded ghost outlines are visible on the LIDAR. Fortresses: Mayans built more of them than expected in the Late Classic Period, and sophisticated ones, too. One has walls over 25 feet high and an Olympic-pool-size reservoir: "In other words, this place was ready for a siege," says Ithaca College archaeologist Tom Garrison. "That is not really the type of conflict that we think about for the ancient Maya." Overall: "Seen as a whole, terraces and irrigation channels, reservoirs, fortifications, and causeways reveal an astonishing amount of land modification done by the Maya over their entire landscape on a scale previously unimaginable," a Tulane researcher says in a statement.