Lasers unveil secrets and mysteries of Angkor Wat

Lidar technology exposes sprawling metropolis surrounding ancient Cambodian temple

Angkor Wat

HIDDEN HISTORY Scientists are retelling the story of Angkor Wat, a medieval temple built in the 12th century by a powerful Khmer ruler.

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Smartphone-toting pilgrims regularly stream into northern Cambodia from all over the world. Their destination: Angkor Wat, a medieval temple that’s famous for massive towers and majestic stone carvings of Hindu gods, spirits and mythological battle scenes. The site, considered the world’s largest religious monument, drew more than 2.3 million visitors in 2014.

Angkor Wat’s sightseers encounter a study in contrasts. This architectural wonder of human civilization ascends skyward, on the verge of being engulfed by nature below. Tourists walk along a path that crosses over a moat and through the temple’s western side, the one entrance cleared of vegetation. Lush forest stops just short of the rest of the structure. Outside the moat, trees, thick ground cover and ponds dominate the landscape.

While adventurous visitors snap pictures, scientists are using high-tech approaches to uncover Angkor Wat’s hidden side, long obscured by all that vegetation. After more than a century of research on the parts of Angkor Wat that are visible with the naked eye, many scientists assumed that the site was a sacred city contained within the bounds of a square moat. But even though its name roughly translates as “temple city,” new finds show that Angkor Wat was not a sacred city at all. It was a gigantic temple connected to residential districts, canals and other structures that stretched beyond the moat and blended into a sprawling city called Greater Angkor, which covered about the same area as Berlin or Columbus, Ohio.

Forest and vegetation surround Angkor Wat (top). A lidar map (bottom) stripped away ground cover to reveal previously unknown archaeological features of the temple and its surroundings. D. Evans et al/PNAS 2013

Angkor Wat’s unveiling by modern laser technology began in April 2012. Archaeologist Damian Evans of Cambodia’s Siem Reap Center and several colleagues made daily helicopter flights for almost two weeks over a 370-square-kilometer area around the temple. The helicopter carried $250,000 worth of special equipment that fired millions of laser pulses every few seconds at the forest below. A small percentage of those pulses zipped in between trees and foliage to the forest floor. The Earth’s hard surface bounced those laser shots back to a sensor on the helicopter. This technique, known as light detection and ranging, or lidar for short, picked up differences in the contours of the land now obscured by jungle. With the findings, researchers could draw a picture of city blocks, residential areas, dried-out ponds and other archaeological remains. Results gleaned from lidar and from new ground-based investigations appeared in the December 2015 Antiquity.

Lidar has been around since the early 1960s. Scientists have used it to measure pollutants in the atmosphere, map shorelines and guide robotic and manned vehicles around obstacles. Lower costs over the last decade have made the technology accessible to archaeologists.

With their laser eye in the sky, Evans and colleagues uncovered big surprises at Angkor Wat. Just beyond one side of the roughly 1.3-kilometer-square moat surrounding the temple, the researchers found six massive and mysterious lines of earth arranged in precise coils — as well as adjacent areas where later canal construction had apparently destroyed two more coiled mounds. These Khmer creations resemble the spiraling paths of labyrinths. Lidar-guided excavations within the moat’s boundary upended ideas about who lived on temple grounds. Rather than religious or political bigwigs, residents were workers who kept the place running. And on-the-ground research found evidence of unexplained towers, built and then demolished during Angkor Wat’s construction, as well as defensive platforms used, perhaps, to fight off invaders.

“It’s an embarrassingly exciting time for archaeologists who study Angkor Wat,” says University of Sydney archaeologist Roland Fletcher. “Researchers have driven and walked over many of these new discoveries for a century.” Fletcher directs the Greater Angkor Project, which combines remote sensing technology with archaeological digs.

If Angkor Wat blended into Greater Angkor’s 1,000 square kilometers of urban sprawl, so did hundreds of other temples and shrines of lesser grandeur built by rulers of Southeast Asia’s Khmer Empire from the ninth to 15th centuries, he says.

“There was nothing like Greater Angkor until the advent of 19th century industrial cities,” says Fletcher, who estimates it held about 750,000 residents in the 12th and 13th centuries. Cities of around 1 million people arose in China by the ninth century, but those metropolises covered one-half or less the area of Greater Angkor. Spread-out cities in the mold of Greater Angkor became more common in the 1800s as trains and cars made long-distance travel easier. But discoveries at Greater Angkor shatter a long-standing assumption that urban sprawl was impossible without mechanical forms of transportation, he says.

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City sprawl

Lidar technology revealed new structures in and around the Angkor Wat temple complex.
Tap red dots for details

Big grids

Carved inscriptions at Angkor Wat describe the structure as the pet project of Suryavarman II, who ruled the Khmer Empire from 1113 until his death in 1149. Serving first as a temple dedicated to the Hindu god Vishnu, Angkor Wat also became a mausoleum for Suryavarman II when he died.

The temple hosted Hindu ceremonies into the 13th century. In the 14th or 15th century, it became a Buddhist temple. Like tourists and scientists, Angkor Wat’s modern-day Buddhist monks had no idea that the site was once so extensive.

Lidar revealed a grid of pathways forming four rectangular blocks, each the size of a small town, that encircle the main temple inside the moat. Earthen mounds and an estimated 250 to 300 ponds — now dried-out depressions on the forest floor — dotted these gridded areas.

Excavations in 2010 and 2013, the latter guided by lidar findings, uncovered remains of modest wooden dwellings and household goods, such as pots and a ceramic cooking device, in the mounds. Archaeologist Miriam Stark of the University of Hawaii at Manoa in Honolulu led that work.

Radiocarbon dating conducted by Stark’s team indicates that houses situated around the ponds appeared at least as early as the sixth century near the eventual site of Angkor Wat. Other residences date to after the 15th century, supporting evidence — including a 17th century Japanese map of the Angkor Wat temple — of the structure becoming a Buddhist religious center.

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No more than 4,500 people lived within Angkor Wat’s moated boundary in the 12th century, Fletcher estimates. His calculation is based on the account of a 13th century Chinese emissary to Greater Angkor, who wrote that one to three families shared each pond in the area around the tample.

Stone inscriptions at nearby Ta Prohm, a late–12th century Greater Angkor temple, record a workforce of 12,640 people that included only about 2,000 on-site residents. Another 66,625 people were described as being “in service” to the temple, delivering food and other supplies.

At roughly twice the size of Ta Prohm, Angkor Wat probably relied on a workforce of about 25,000, Fletcher suspects. An additional 125,000 people must have shuttled in supplies. Each Greater Angkor temple apparently supported a vast economy.

If that’s true, it makes sense that crisscrossing roads forming residential blocks fan out far beyond Angkor Wat’s moat on the 2012 lidar map. “Urban grids stretched beyond temple walls into the hinterlands,” Evans says.

Lidar data showed a comparable street network at a nearby 12th century temple, Angkor Thom. Here too, “to our complete surprise, the layout of houses and streets continues beyond the moated confines of the temple,” says archaeologist Charles Higham of the University of Otago in New Zealand. He studies Greater Angkor and is familiar with the lidar results.

Religious and political big shots lived somewhere in the urban sprawl outside Angkor Wat and other temples, Fletcher suspects. Why they did so is unclear.

Mystery coils

Finding that Angkor Wat extended far beyond its moat was unexpected. But a discovery just south of the temple’s moat was unprecedented.

Lidar unveiled long banks of earth that formed six well-preserved coiled or spiral-shaped patterns. Each formation is roughly 1 kilometer long and 0.5 kilometers wide, or about 10 times the length and width of a football field. The precise alignment of these earthworks with the moat suggests they were assembled around the time of Angkor Wat’s construction.

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A ground survey in late 2012 and early 2013 determined that the coils consisted of 18-meter-wide sandbanks separated by 12-meter-wide channels. After using laser maps to locate the spirals on the ground, investigators walked through the channels searching for signs of human activity. They found no pottery or other artifacts to suggest that anyone had lived or worked there.

“Nothing like the shape and design of these spirals has been seen anywhere else,” Fletcher says.

Suryavarman II’s reasons for building the coils are unknown. These mounds might have served as raised fields for growing herbs used in temple rituals. Or the coils might have been sites of formal gardens that would have been unrivaled in size and complexity until the construction of 18th century palace gardens in Europe, Evans suggests.

Rainwater could have flowed through channels between the coils. Evans doubts, however, that enough water collected to support farming.

It’s also possible that Angkor Wat’s spiral structures held special spiritual meaning for Khmer people and had no practical use. Yet Fletcher says that nothing resembling these coiled forms appears in Hindu writings and art.

Angkor Wat statues depict Hindu gods and goddesses and tell stories from Hindu scriptures. Appie Verschoor/Flickr (CC BY-SA 2.0)

After what must have been several decades of construction coinciding with work on the temple, “the spirals may never have been completed and might never have become operational,” Evans says.

Researchers are now examining fossilized pollen recovered from the coils for signs of cultivation or gardening.

Buried towers

Another surprising discovery at Angkor Wat comes not from lidar but from ground-penetrating radar. While lidar reveals characteristics of the ground’s surface covered by vegetation, ground-penetrating radar can detect objects buried deep under several dozen meters or more of soil.

In December 2009, a team led by archaeologist Till Sonnemann of Leiden University in the Netherlands dragged a wheeled device reminiscent of a lawn mower over Angkor Wat’s vast outer section for two weeks. High-frequency radio waves emitted by the contraption bounced off buried objects, signaling locations of possible archaeological remains.

Sonnemann was looking for remnants of houses or administrative buildings from the 12th century or later. Something far more intriguing turned up.

At Angkor Wat’s western entrance, where visitors enter the grounds of the temple, the radar machine identified what looked like the foundations of eight towers. Excavations in 2010 and 2012 confirmed their existence. Foundation remains lie roughly 21 meters belowground, about 10 times as deep as an Olympic swimming pool.

Each cross-shaped foundation, held in place by walls made of a reddish rock called laterite, had a square central section bound by porches jutting out on each side. Intact shrine towers from other Khmer Empire sites, which were dedicated to various gods, feature side porches.

Angkor Wat’s former towers were intentionally destroyed, Sonnemann says. Radiocarbon dating of burned wood from the foundations suggests the towers were built around the time that work started on Angkor Wat. Demolition occurred when Angkor Wat’s outer wall and western gate were completed, Sonnemann suspects.

Perhaps 12th century residents of Greater Angkor used the gateway towers as a temporary religious shrine to the Hindu god Vishnu while erecting Suryavarman II’s temple, which was dedicated to the same deity, Sonnemann says. Of the eight towers, four formed a square that stood within a larger square formed by the four others. Angkor Wat itself features four towers arranged in a square around a central tower.

The gateway towers may have served as an outline of Angkor Wat’s permanent towers, Sonnemann speculates. But they were not identical. Remote sensing identified no remnants of a central tower at the western entrance.

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A digital reconstruction from radar imaging shows the position of towers (lower left, bright yellow) that were built and intentionally destroyed during construction of the temple (upper right). T. Sonnemann, ETH Zurich

Wall defenses

Tourists and researchers have long gazed at Angkor Wat’s impressive stone outer wall without realizing it holds clues to warfare between the Khmer Empire and regional foes, says archaeologist David Brotherson of the University of Sydney.

Archaeologist Christophe Pottier of the Bangkok Center in Thailand first noticed holes that had been intentionally carved in parts of the wall constructed later. When Brotherson took a closer look, he speculated that those openings once supported wooden platforms and fences.

Angkor Wat’s defenders stood on the platforms and positioned themselves between the fences to repel attacks by nearby Thai kingdoms, Brotherson proposes. Those confrontations probably took place sometime between the late 13th and early 17th centuries.

Brotherson studied 6,257 wall cavities. Most consist of groups of seven square holes, at the same height, running across the inside of the wall near its top. Sets of holes, notches and grooves also run across adjacent areas on top of the wall.

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Holes (circled) in the inside of Angkor Wat’s largest enclosure wall may have held supports for wooden platforms where defenders could stand to fight off foreign attackers. D. Brotherson/Antiquity 2015

These alterations appear at spots where six nearly 20-meter-wide gaps in the wall — which probably framed wooden gates — were later filled in with stone blocks. Differences in detailing and finish distinguish original masonry from filled-in sections.

Holes on the inner part of the wall held wooden beams that supported platforms about 3.5 to 4 meters above the ground that must have had stairs at each end, Brotherson says. Angkor Wat’s fighters would have stood on platforms while raining down arrows or other weapons on attackers. Wall-top holes probably held fence posts for additional protection, he suspects. Yet researchers have found no arrowheads or other weapons and no evidence of military damage to Angkor Wat, such as wall marks from catapulted boulders or remnants of torched wooden buildings.

Still, it’s more likely that the wall’s inside holes held platforms for temple defenders to stand on than wooden roofs that sheltered people or animals underneath, Brotherson says. Gateway spaces would not have been filled in simply to construct shelter roofs, he contends. And roofs would not have required holes carved on top of the wall.

“There are no historical references to defensive fortifications at Angkor Wat,” Fletcher says. “Sometimes archaeology tells us things that history cannot.”

Tropical trajectories

Lidar’s bird’s-eye view of forest floors has guided archaeologists not only to lost parts of Angkor Wat, but to a greater appreciation of similarities between Greater Angkor and other ancient tropical cities.

Lidar surveys of west-central Belize in 2009 and 2013 showed that the ancient Maya city of Caracol sprawled across now-forested landscape much as Greater Angkor did. A dense urban area, incorporating agricultural fields into a planned city, spread 10 kilometers in all directions from central Caracol in 650. Researchers estimate that more than 140,000 people lived at Caracol (SN: 12/15/12, p. 14).

Laser maps revealed farming terraces, housing tracts and roads leading to public plazas that ranged far beyond Caracol’s urban center. Anthropologists Arlen and Diane Chase of the University of Central Florida in Orlando directed lidar research at Caracol.

At 1,000 square kilometers, Greater Angkor covered a much larger area than Caracol. “Lidar surveying has just begun, but we now know that Maya cities were shrimps compared with mighty Angkor,” says Yale University anthropologist Michael Coe, a long-time investigator of the Maya and other ancient American civilizations.

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Nothing like Angkor’s street grids stretching with geometric precision toward the horizon appears at Caracol, adds Coe, who is familiar with work at both sites.

Still, common factors inspired the rise and fall of Greater Angkor, Maya urban centers such as Caracol and the tropical city of Anuradhapura in Sri Lanka between the ninth and 16th centuries, Fletcher and two colleagues concluded in the October 2015 Antiquity.

Rulers in each region directed the construction of reservoirs and canals that allowed spread-out cities to flourish. Effective water management cemented the power of kings by enabling masses of farmers to make a steady living.

Severe periods of drought, indicated by analyses of tree rings and lake sediments, strained water supplies in each tropical setting, Fletcher says. Periodic monsoon rains in Southeast Asia and Sri Lanka added insult to injury, overwhelming reservoirs and damaging canals.

Unable to supply enough water, political systems at Greater Angkor and other tropical cities crumbled, Fletcher argues. But societies didn’t vanish. Farmers reorganized into smaller communities based near coasts and along major rivers. Cultivation continued in fields that had once been part of massive cities.

Laser quest

Fletcher’s rise-and-disperse scenario for ancient tropical cities is being put to a broader test. From March to May 2015, a second set of lidar flights expanded laser mapping of northern Cambodia to a total area of 2,000 square kilometers. Researchers want to see if, like Angkor Wat, other ancient temples in the region sat in the center of dispersed settlements tied together by reservoirs, canals and ponds. Archaeological investigations based on the new lidar data are under way. Evans plans to announce initial findings in June.

Over the next 10 to 15 years, lidar technology will become smaller and cheaper, Evans predicts. Laser-wielding drones will replace lidar-toting helicopters. But laser mapping is already a game changer for tropical archaeology.

Around Angkor, “the impact of lidar data is like turning on a light after groping in the dark for over a century,” says archaeologist John Miksic of the National University of Singapore.

Suryavarman II, a politically ambitious warrior who tried to expand his kingdom by launching wars and allying himself with Imperial China, would surely celebrate Angkor Wat’s laser revival if he was around today. It’s a revival of sorts for the once-mighty king, as well. 

This article appears in the May 14, 2016, issue of Science News with the headline, “Cambodia’s hidden city: Laser technology reveals surprising sprawl around Angkor Wat.”


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