CANCUN, Mexico — By bombarding a patch of the Honduran rainforest with laser pulses, archaeologists have discovered structures that could be a part of a lost city — or two.

In spring 2012, scientists from the National Center for Airborne Laser Mapping (NCALM), based at the University of Houston, loaded a plane with a state-of-the-art lidar system and took it down to Honduras. Lidar bounces billions of laser pulses off of the forest and measures the time they take to return. Though most of the pulses reflect off vegetation, some small fraction reaches the ground. Researchers can thus build up a map of the surface by mathematically stripping away the canopy of tree leaves (shown at right).

Lidar has been used to calculate biomass in the Amazon and to hunt for extra structures at Stonehenge. In the dense forests of Central America, though, lidar “is like rewriting history,” says Christopher Fisher, an archaeologist at Colorado State University in Fort Collins. “We have just huge black holes on the map about which we know very little.”

The NCALM survey flew right over one of those black holes. Directed by Los Angeles filmmaker Steve Elkins, who is making a documentary about the project, the lidar plane visited four targets in the Mosquitia rainforest. All were possible locations of a long-sought ruin known as the Ciudad Blanca or White City.

“The White City is quite the legend in Honduras,” says NCALM scientist Juan Carlos Fernandez Diaz, a native Honduran. Explorers have sought this “lost city” for decades, although many archaeologists believe it may be a myth or perhaps an amalgam of other Mesoamerican cities.

In May 2012, the NCALM team presented its findings to the Honduran government. On 15 May 2013, at the American Geophysical Union Meeting of the Americas in Cancun, Elkins showed previously unreleased images from the survey. They include regularly spaced mounds and other linear features that make up at least two Mesoamerican cities, says Fisher. (Shown, at right, are features at the site known as T3; the rectangular shape in the lower centre is approximately 50 metres long.)

“We’re trying to identify the densest amount of features so we can go there and look at them,” says Stephen Leisz, a geographer at Colorado State. Elkins is planning to helicopter into the area in November to target places to send archaeologists on the ground.

Lidar surveys are expensive: The team, funded by filmmaker Bill Benenson, has already spent close to half a million dollars on the Honduran project. And there’s no guarantee what the team might find when they do visit; they don’t even know the age of the potential structures they have spotted. The group is keeping the location a secret.

But Fisher, at least, has reason to hope that lidar surveys might become a common tool for archaeologists. He worked for years at the Mesoamerican city of Angamuco, in central Mexico. He eventually coughed up $38,000 for a lidar survey of 9 square kilometres, and it revealed more than 20,000 architectural features in the city’s urban core.

They include a pyramid that Fisher had missed by just 10 metres on a previous ground survey when he walked right past the jungle-covered structure.

Images: UTL Scientific, LLC