How a 3,000-year-old Maya city stayed hidden until now

HostYou can look at a map of the deep jungle and see what looks like a big green blank space where nothing much ever happened. But it turns out we have been walking right over whole cities for hundreds of years without ever knowing they were there. I mean, how does a place with huge stone pyramids stay lost when it's just a short walk from a main road and a local farm?

GuestIt's a bit of a shock to the system. This city, which we now call Valeriana, was actually found by a student sitting at a desk in his office. He was just browsing the web, looking at some old data that a group of nature experts had put online years ago. He was way down on the second page of a Google search when he saw a shape in the ground that didn't look natural. It turned out to be a massive sprawl that likely held thirty thousand people at its height. It's the second biggest Maya site we have ever found, and we only spotted it because of a lucky click of a mouse.

HostThat sounds like a dream for any researcher, but how do you miss a whole city if you're already out there in the woods?

GuestThe jungle is like a thick green blanket that hides everything. If you're standing at the bottom of a Maya pyramid today, you might not even know it. Over a thousand years, trees grow over the stone, dirt piles up, and the whole thing just looks like a steep hill. Unless you have a shovel and a few months to dig, you would just walk right past it. But we have this new tool now called LiDAR. It stands for light detection and ranging. Basically, a plane flies over the forest and shoots out millions of laser pulses every second. These little beams of light hit the ground and bounce back up. Some hit the leaves, some hit the branches, but some sneak through the gaps and hit the dirt.

HostSo the lasers sort of peel back the trees like an orange skin?

GuestIn a way, yeah. A computer takes all those millions of points and filters out everything that's a tree or a bush. What you're left with is a map of the bare ground. It's like turning off the forest. When this student looked at that map, he saw these clear, sharp lines. He saw squares, long flat roads, and two big pyramids. These weren't hills made by nature. They were buildings made by people. And the wildest part is that this city wasn't out in some untouched wilderness. It was right next to a highway and a town where people have been living and farming for decades. They were literally growing crops on top of a lost world.

HostIt feels like we're finding out that the Maya world was way more crowded than we used to think.

GuestMuch more. We used to think the Maya lived in small, spread out groups with a lot of empty space between them. We thought they just had a few big ritual spots. But these laser maps show us that the whole landscape was lived in. Valeriana had houses packed together, big plazas for markets, and even a court for their famous ball games. They had also built these massive walls to hold back water so they could farm the hills. It shows a level of engineering that we usually only link with places like ancient Rome. They weren't just living in the jungle. They had completely reshaped the earth to fit thousands and thousands of people.

HostIs there a reason it took so long to use these lasers on this specific patch of land?

GuestIt's mostly a matter of cost. Flying a plane with a laser scanner is very expensive. But the data this student found was actually paid for by a group that wanted to study how much carbon was in the trees. They didn't care about old ruins at all. They just wanted to measure the forest. Archaeologists are now realizing that there's a huge pile of this kind of data just sitting around on government servers. It's like a digital treasure map that nobody has bothered to read yet. We have mapped about ten times more land in the last decade than we did in the whole century before that.

HostSo there could be more of these massive cities just waiting in folders on someone’s hard drive?

GuestThere almost certainly are. We have only looked at a tiny slice of the Maya lowlands with these lasers. Every time we point the scanner at a new spot, we find something big. It's changing how we see the end of their culture too. We used to ask why they just disappeared. But when you see how packed these cities were, you start to see the pressure they were under. If a long dry spell hit, a city of thirty thousand people in the middle of a forest is in big trouble. They might have just run out of ways to feed everyone.

HostIt's a bit humbling to think that even with our planes and satellites, we can still lose track of a place that big.

GuestWe're finding that the map of the world still has plenty of holes in it.

HostThe woods are still very good at keeping secrets, even when we're standing right on top of them.