(curious music) - I like to think back to 3 million years ago when this was forming and those big, big cavities were being dissolved away by little tiny microbes that are 1/100 the size of one of your hairs - [Narrator] Below the Chihuahuan Desert in southeastern New Mexico lies an extensive system of limestone caves.

Among them, the pristine Lechuguilla Cave and the famed Carlsbad Cavern, home to one of North America's largest underground chambers.

But there's more to this subterranean world than gigantic geological formations.

It's also ground zero for a microbial war that's been raging for thousands of years right beneath our feet.

(serious music) - People knew of this cave for a long time.

We have rock art left by the entrance of native people.

There's places where they roasted food right at the entrance of the cave as well.

Right around 1898, Jim White, a young cowboy, was working at a ranch.

One day, he saw something different.

Up on the ridge top was a big black, billowing cloud of smoke.

So as any 16-year-old would, he rode straight for it.

But he found no lava, no magma neither, but hundreds of thousands, if not millions of Brazilian free-tailed bats.

He realized that any cavity that could hold that quantity of bats had to be massive.

And this started a lifelong love affair.

- Compared to almost all the other cave systems in this area, it is very, very large.

In fact, the big room in Carlsbad Caverns is one of the largest underground chambers in North America - [Nick] Where you get to see things like the Rock of Ages.

There's the Hall of Giants, the chandeliers, below them the totem poles, massive skinny stalagmites, 30, 40 feet tall.

(subdued music) - One of the great debates about cave formation is, is it geology or is it microbiology?

And I think the real answer is it's a combination of both of these things.

- Rather than the way most caves form, which is from the top down with carbonic acid.

This cave formed from the bottom up.

There is the release of hydrogen sulfide gas.

It's what we call rotten egg gas so it stinks, but it's got extra electrons that can be taken from it to provide energy.

And so when they hit the oxygenated area, microbes at that region oxidize the hydrogen sulfide to sulfuric acid and that dissolves away the carbonate.

And because it's such a strong acid, that's why you've got these amazingly huge cavities.

But what about all the decorations?

What you're seeing in a giant formation like this is a lot of precipitation.

Sometimes microbes help with this.

So we eat and we put on weight, the microbes eat and they put on mineral.

So it's kind of a partnership between the microorganisms, the air, the geochemistry, the environment, all those different things that help make the formations.

(curious music) - One of the great things about doing microbiology in caves is that caves are a really extreme environment.

If you think about it, there's not a lot of energy in a cave.

So what are the microbes going to eat?

The answer is that they have to compete really strongly against each other.

And so it's continual microbial warfare.

What does this mean?

That when a microbe gathers some energy, it's gonna store it in a form that produces a compound that will kill lots of other microbes which could potentially cause diseases, and that stuff is antimicrobial.

So it's a really exciting place to study microbiology.

- One of the questions that we ask when we look for microbes in caves is what they can tell us about what's called biosignatures.

When we go to Mars, or somewhere else, we have to know, how will we recognize that is life?

All these different morphologies have microbes buried in them.

So we're trying to see how we will recognize that that's a good place to sample.

- So there's a surprising amount of crossover between NASA research and research in this park.

If you think about it, we don't know what microbial life might be like on planets other than our planet.

And we also want to be able to prevent taking microbes from Earth to those other planets 'cause we don't wanna contaminate those other planets.

So researchers will come here and they will look for all the extreme microbes that have managed to live in this really energy-starved location, and they'll find out how they might survive, and they also might find out like how to kill them to prevent them from contaminating any spacecraft we send to other planets.

(gentle inquisitive music) There are only a few places for true exploration: space, the deep oceans, and caves.

We have a special obligation to protect these places, especially if you're the first person to go to a particular cave passage or cave because you might be disturbing something that has been undisturbed for thousands or even millions of years.

(gentle inquisitive music continues)