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.
It was my first glimpse of the valley. The photos, taken with a shaky telephoto through scratched Plexiglas, were not clear; but they showed two squarish white objects that looked like the tops of carved limestone pillars, opening into an area of low vegetation that was square in shape. The feature was on a brushy floodplain in the upper end of the valley. Everyone crowded around the laptop, squinting, pointing, and talking excitedly, trying to make sense of the pixilated images that were so tantalizingly ambiguous—they could be pillars, but then again they could be trash dropped from a plane or even the tops of two dead tree stumps.
I pleaded to accompany the third and final flight over T1, despite the logistical issues it posed. There was no room in the plane, but after some discussion, Chuck Gross agreed that he might be able to clear out a tiny space for me to crouch in. He warned me it would be mighty uncomfortable over six to seven hours of flying time.
On May 4, we arrived at the airport as the sun was just rising above the curve of the ocean, the plane throwing an Edward Hopper shadow across the tarmac. The soldiers guarding the plane greeted us sleepily. Now that I was about to be a passenger I looked at the plane more attentively, and I did not like what I saw.
“What’s with that oil streak?” I asked Chuck.
“Don’t worry about that,” he said. “I’m topping it off every day. In one flight it won’t lose enough to make a difference.”
As I crawled on board, my dismay deepened. The interior of the Cessna, once a rich velvetized fabric in burgundy, was now worn, greasy, and faded; much of the inside appeared to be held together with duct tape. It smelled of Eau de Old Car. Parts of the plane had been sealed with acrylic caulk, now peeling out in strings. As I tried to maneuver around the giant lidar box into the micro-space provided, I bumped my elbow into a panel, which fell off.
“No worries, that always happens,” said Gross, reseating it with a blow from his fist.
I marveled that a plane as unsafe and decrepit as this one looked would be used to carry a million-dollar scientific instrument. Chuck firmly disagreed. “No, sir,” he said. “This plane’s a perfect platform for the job.” He assured me the 337 Skymaster was a “classic,” and a “great little aircraft.” Unlike a King Air or a Piper Navajo, he said, this craft was ideal, with a fuel efficiency that would allow us to spend “six hours on station.” Even though it was forty years old, it was “totally dependable.”