NARRATOR：

Listen to part of a lecture in an archaeology class.

[as though answering a student’s question] Sure, sometimes we do just stumble onto an important find when doing field research.

But usually we've got at least a vague idea of where to look, and with new technology...

Uh, OK, here's a story that illustrates what I mean.

It's about the Mayans, who, as you remember, flourished in Central America and had a culture that was quite advanced in art...architecture...astronomy...

We know that, despite regular droughts and poor soil, their numbers grew into the millions over the centuries... until about twelve hundred years ago, when their entire civilization just seemed to disappear.

And we're not sure why.

OK, so an archaeologist named William Saturno goes looking for ancient Mayan ruins in Guatemala, near a town called San Bartolo.

And after several days of extremely difficult hiking through the thick rain forest, Saturno stops to rest in the shade... and finds himself sitting in what turns out to be an ancient Mayan temple- a pyramid twenty-five meters high!

And inside, on the walls of this temple, Saturno finds some ancient writing and also this enormous mural with elegant figures depicting a Mayan myth of the creation of the world.

[emphasize] And it's all painted on plaster that's over two thousand years old, which makes it the oldest Mayan artwork ever found, at least in good condition!

And in fact, one of the most perfectly preserved. An extremely important find.

MALE STUDENT：

[not overly dramatic] Wow... Do you have a picture of it?

FEMALE PROFESSOR：

Now hang on- I don't...there's a point I wanna make here.

It happens that someone at NASA, the United States space agency, reads about Saturno's discovery and gets very excited cause the space agency has just produced some images of this area using a technique called "remote sensing."

That's when instruments on planes and satellites survey areas on the ground.

And the newest twist on remote sensing-quite new- is "infrared imaging."

Instead of taking regular photographs, the satellite cameras take pictures using infrared light, which is invisible to the human eye.

But computers can then process these images so our eyes can see them.

Using infrared imaging, the satellite-based remote-sensing instruments revealed what turned out to be traces of water-storage systems and canals... canals that the Mayans built to irrigate their parched soil, which helps explain how the Mayans could feed such a large population.

The infrared images also revealed ancient roadways that had tied Mayan cities together.

So people at the space agency figure Saturno'll be interested, and they send him this infrared image of the area near San Bartolo where the pyramid temple was found.

Now, this is a false-color image based on an infrared photo, so the greens of the jungle are shown mostly as blue and red.

But notice also the spots of greenish-yellow scattered here and there.

These indicate significant discoloration in the vegetation, at least as it appears to infrared cameras.

And Saturno notices that some of that discoloration's located in exactly the spot where he found the pyramid temple.

So he figures, hey, maybe some of those other yellow spots are worth investigating...

Well, long story short, he checks out three different spots where the photo shows a discoloration and finds an ancient Mayan site, overgrown with vegetation, at every single one.

Further exploration shows a perfect correlation between yellow spots on the infrared image and Mayan ruins hidden in the jungle.

MALE STUDENT：

So what caused those spots to look different?

FEMALE PROFESSOR：

Well, Saturno believes the limestone and lime plasters that the Mayans used to build their structures... over time, uh, this limestone decayed and seeped into the soil, and changed the soil's chemistry.

Then calcium carbonate from the decaying lime plaster might've been taken up by the roots of the trees growing there- up into their leaves- and made them give off infrared light much more brightly than the surrounding vegetation.

And infrared sensing technology can detect this.

MALE STUDENT：

So, like, is Guatemala the only place where archaeologists have used remote sensing?

FEMALE PROFESSOR：

No, this technique's been used in other parts of Central America too, and also in Brazil, Bolivia, Cambodia...

It can be used anywhere the rainforest has obscured ancient ruins.

And the results can be amazing...like, another Mayan temple that Saturno found thanks to remote sensing, he'd walked right by it every day for five years and had no idea it was there- until he saw an infrared image of the area.