The technology of lidar was developed soon after the discovery of lasers in the early 1960s. Put simply, lidar works like radar, by bouncing a laser beam off something, capturing the reflection, and measuring the round-trip time, thereby determining the distance. Scientists quickly realized its potential as a mapping tool. Both the Apollo 15 and 17 missions carried a lidar machine on the orbiter, which mapped swaths of the moon’s surface. The Mars Global Surveyor, a satellite orbiting Mars, also carried a lidar machine, which bounced laser beams off the surface of Mars ten times per second. Over its ten-year mission, from 1996 to 2006, the Surveyor created a prodigiously accurate topographic map of the Martian surface, one of the supreme mapping projects of human history.

There are three types of lidar instruments: spaceborne, aerial, and terrestrial. On earth, aerial lidar has been used in agriculture, geology, mining, tracking glaciers and ice fields for global warming, urban planning, and surveying. It had numerous classified uses in the wars in Iraq and Afghanistan. Terrestrial lidar is currently being tested in self-driving vehicles and “intelligent” cruise control, which use lidar to map the ever-shifting environment around a car moving down a roadway, as well as to make detailed three-dimensional maps of rooms, tombs, sculptures, and buildings; it can re-create digitally, in incredibly fine detail, any three-dimensional object.

The target sites of T1, T2, and T3 would be mapped with this Cessna, the same one used over Caracol. As the plane is flown in a lawnmower pattern over the jungle, the lidar device fires 125,000 infrared laser pulses a second into the jungle canopy below and records the reflections. (The laser pulses are harmless and invisible.) The time elapsed gives the exact distance from the plane to each reflection point.

The lidar beam does not actually penetrate foliage. It does not “see through” anything in fact: The beam will bounce off every tiny leaf or twig. But even in the heaviest jungle cover, there are small holes in the canopy that allow a laser pulse to reach the ground and reflect back. If you lie down in the jungle and look up, you will always see flecks of sky here and there; the vast number of laser pulses allow lidar to find and exploit those little openings.

The resulting data is what lidar engineers call a “point cloud.” These are billions of points showing the location of every reflection, arranged in 3-D space. The mapping engineer uses software to eliminate the points from leaves and branches, leaving only bounce backs from the ground. Further data crunching turns those ground points into a hill-shade picture of the terrain—revealing any archaeological features that might be present.

The resolution of the lidar image is only as good as how well you keep track of the position of the plane flying through space. This is the greatest technological challenge: In order to achieve high resolution, you need to track the plane’s position in three dimensions during every second of flight to within an inch. A standard GPS unit using satellite links can only locate the plane within about ten feet, useless for archaeological mapping. The resolution can be refined to about a foot by placing fixed GPS units on the ground underneath where the plane will be flying. But an airplane in flight is being bounced around by turbulence, subjected to roll, pitch, and yaw, which not even the finest GPS unit can track.

To solve this problem, the lidar machine contains within it a sealed instrument that looks like a coffee can. It contains a highly classified military device called an inertial measurement unit, or IMU. This is the same technology used in cruise missiles, allowing the missile to know where it is in space at all times as it heads toward its target. Because of the IMU, the lidar machine is listed as classified military hardware, which cannot leave the country without a special permit, and even then only under highly controlled conditions. (This is another reason why there was a long lag-time in the use of lidar at Third World archaeological sites; for years the government prevented the IMU from being used outside the country in civilian applications.)

Aerial lidar can achieve a resolution of about an inch, if there is no vegetative cover. But in the jungle, the canopy causes the resolution to drop precipitously, due to many fewer pulses reaching the ground. (The fewer the pulses, the lower the resolution.) The Belizean rainforest around Caracol, where the Chases had used it in 2010, is thick. But it doesn’t come anywhere near the density of Mosquitia.

The first lidar flight over T1 took off the next day, May 2, 2012, at 7:30 a.m., with Chuck Gross at the controls and Juan Carlos Fernández acting as navigator and running the lidar machine. We all went to the airport to see the plane off, watching it rise into the Caribbean skies and wink into the blue across the Gulf of Honduras, heading for the mainland. It would take three days to map the twenty square miles of T1. If all went well, we would know in four days if T1 held anything of interest. After that, the plane would shift to T2 and T3.

The plane returned from its first mission in late afternoon. By nine in the evening Sartori confirmed that the data was clean and good; the lidar machine was operating flawlessly and they were getting enough ground points through the forest canopy to map the underlying terrain. While he had no images yet, he saw no technical reason why we wouldn’t get detailed terrain maps.

After the second day of flying, on May 3, Juan Carlos came back with intriguing news. He had seen something in T1 that didn’t look natural and had tried to photograph it through the windows of the Skymaster. We gathered in his bungalow to look at the photos on his laptop.