OFFICIAL16 According to the professor, what substance found in surface water is important for the formation of typical limstone caves?

[00:00.00]Narrator: Listen to part of a lecture in a geology class.
[00:06.14]MALE PROFESSOR: Now, there are some pretty interesting caves in parts of the western United States, especially in national parks; [00:13.66]there's one park that has over a hundred caves, including some of the largest ones in the world. [00:19.98]One of the more interesting ones is called Lechuguilla Cave.[00:24.79]Lechuguilla’s been explored a lot in recent decades. [00:30.33]It's a pretty exciting place, I think. [00:32.48]It was mentioned only briefly in your books, so can anyone remember what it said? Ellen?
[00:38.80]Female Student: It's the deepest limestone cave in the U.S.[00:43.02]MALE PROFESSOR: That’s right—it's one of the longest and deepest limestone caves not just in the country, but in the world. [00:50.51]What else?[00:51.80]Female Student: Well, it was formed because of sulfuric acid, right?
[00:55.67]MALE PROFESSOR: That's it. Yeah, what happens is, you have deep, underground oil deposits. And there are bacteria... [01:03.29]here, let me draw a diagram.
[01:06.24]Part of the limestone rock layer is permeated by water from below. Those curly lines are supposed to be cracks in the rock. [01:14.75]Below the water table and rock is oil. [01:19.15]Bacteria feed on this oil, and release hydrogen sulfide gas. [01:24.11]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.
[01:33.78]And when hydrogen sulfide reacts with the oxygen in the water, the result of that is sulfuric acid. OK? [01:42.75][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. [01:56.56]Yes, Paul?
[01:58.79]Male student: So that water, it's not flowing, right? It’s still?
[02:02.85]MALE PROFESSOR: Yes! So there’s two kinds of limestone caves. [02:06.82]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. [02:17.38]It’s the moving water itself that wears away at the rock and makes passageways.
[02:22.52]Also, in surface water, there's a weak acid, carbonic acid—not sulfuric acid, but carbonic acid, that helps dissolve the rock.[02:34.12]With a little help from this carbonic acid, moving water forms most of the world’s limestone caves.
[02:41.80] 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. [02:52.88]This flowing water carved out the caves and the structures inside them.[02:57.84]Male student: But not Lechuguilla?[02:59.09]MALE PROFESSOR: Dry as a bone. [03:00.61] 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. [03:13.75]In fact, there’s no evidence that flowing water has ever gone in or out of the cave. [03:19.84]So it's like a maze, you have passageways all around. [03:24.07]There're wide passages, narrow ones, at all different depths … like underground tunnels in the limestone.
[03:30.31]And since they were created underground and not from flowing surface water, not all of these passageways have an opening to the outside world.[03:39.09]And there’s other evidence that flowing water wasn't involved in Lechuguilla. [03:45.15]We've said that sulfuric acid dissolves limestone, right? [03:49.38]And forms the passageways? [03:51.42] What else does sulfuric acid do? Paul.[03:56.43]Male student: It leaves a chemical residue. Um, 
[03:59.39]Female Student: Gypsum, right?[04:00.43]MALE PROFESSOR: Yep. You'll find lots of gypsum deposited at Lechuguilla. [04:04.82]And, as we know, gypsum is soluble in water. So if there were flowing water in the cave, it would dissolve the gypsum. [04:13.28]This is part of what led us to the realization that Lechuguilla is in that small group of waterless caves.[04:20.60]And, Lechuguilla is pretty much dormant now. [04:24.52]It's not really forming anymore …
[04:26.92]but there's other ones like it, for example in Mexico, that are forming, [04:33.02]and when cave researchers go to explore them, they see—and smell—the sulfuric acid and gases at work. [04:41.42]Whew! It’s something else! Think of rotten eggs. [04:45.72]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?[04:55.84]Male student: Yeah, how about what these caves look like, uh, on the inside?
[04:59.34]MALE PROFESSOR: Oh, well, the formations are something. [05:02.79]And there's such a variety there, like nothing anywhere else in the world! [05:07.07]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. [05:14.75]It's pretty impressive.