NARRATOR：

Listen to part of a lecture in a geology class.

MALE PROFESSOR：

Now, there are some pretty interesting caves in parts of the western United States, especially in national parks; there’s one park that has over a hundred caves, including some of the largest ones in the world.

One of the more interesting ones is called Lechuguilla Cave.

Lechuguilla’s been explored a lot in recent decades. It’s a pretty exciting place, I think.

It was mentioned only briefly in your books, so can anyone remember what it said?

Ellen?

FEMALE STUDENT：

[upspeak]It’s the deepest limestone cave in the U.S.

MALE PROFESSOR：

That’s right—It’s one of the longest and deepest limestone caves not just in the country, but in the world. What else?

FEMALE STUDENT：

Well, it was formed because of sulfuric acid, right?

MALE PROFESSOR：

That’s it. Yeah, what happens is, you have deep, underground oil deposits. And there are bacteria... here, let me draw a diagram.

Part of the limestone rock layer is permeated by water from below.

Those curly lines are supposed to be cracks in the rock. Below the water table and rock is oil.

Bacteria feed on this oil, and release hydrogen sulfide gas.

This gas, this hydrogen sulfide, rises up and mixes with oxygen in the underground water that sits in the cracks and fissures in the limestone.

And when hydrogen sulfide reacts with the oxygen in the water, the result of that is sulfuric acid.

OK? [short pause]Sulfuric acid eats away at limestone—very aggressively—so you get bigger cracks, and then passageways being formed along the openings in the rock. And it’s all underground.

Yes, Paul?

MALE STUDENT：

So that water, it’s not flowing, right? It’s still?

MALE PROFESSOR：

Yes! So there’s two kinds of limestone caves.

In about 90 percent of them, you have water from the surface, streams, waterfall, or whatever.

Moving water that flows through cracks found in the limestone.

It’s the moving water itself that wears away at the rock and makes passageways.

Also, in surface water, there’s a weak acid, carbonic acid—not sulfuric acid, but carbonic acid, that helps dissolve the rock.

With a little help from this carbonic acid, moving water forms most of the world’s limestone caves.

When I was researching this for a study a few years ago, I visited a couple of these typical limestone caves.

They were all very wet, you know? From streams and rivers.

This flowing water carved out the caves and the structures inside them.

MALE STUDENT：

But not Lechuguilla?

MALE PROFESSOR：

[matter-of-factly] Dry as a bone.[hesitates] Well, that might be a bit of an exaggeration, but it’s safe to say that it’s sulfuric acid, and not moving water, that formed Lechuguilla Cave, and those few other ones like it

In fact, there’s no evidence that flowing water has ever gone in or out of the cave.

So it’s like a maze, you have passageways all around.

There’re wide passages, narrow ones, at all different depths… like underground tunnels in the limestone.

And since they were created underground and not from flowing surface water, not all of these passageways have an opening to the outside world.

And there’s other evidence that flowing water wasn’t involved in Lechuguilla.

We’ve said that sulfuric acid dissolves limestone, right? And forms the passageways?

What else does sulfuric acid do? Paul.

MALE STUDENT：

It leaves a chemical residue. Um, [interrupted]

FEMALE STUDENT：

[interrupting]Gypsum, right?

MALE PROFESSOR：

Yep. You’ll find lots of gypsum deposited at Lechuguilla.

And, as we know, gypsum is soluble in water.

So if there were flowing water in the cave, it would dissolve the gypsum.

This is part of what led us to the realization that Lechuguilla is in that small group of waterless caves.

And, Lechuguilla is pretty much dormant now.

It’s not really forming anymore… but there’s other ones like it, for example in Mexico, that are forming, and when cave researchers go to explore them, they see—and smell—the sulfuric acid and gases at work.

Whew! It’s something else! Think of rotten eggs.

And it’s not just the smell. Explorers even need to wear special masks to protect themselves from the gases in these caves.

OK. Paul?

MALE STUDENT：

Yeah, how about what these caves look like, uh, on the inside?

MALE PROFESSOR：

Oh, well, the formations are really something.

And there’s such a variety there, like nothing anywhere else in the world!

Some of them are elaborate-looking, like decorations. And a lot of them are made of gypsum, and can be up to twenty feet long.

It’s pretty impressive.