Thanks to Linode, a top-rated  cloud computing company, for supporting this episode of SciShow.

Head to linode.com/scishow to learn more and get a $100 60-day credit  on a new Linode account. [♪ INTRO] Caves are fascinating. On the one hand, they can be exciting,  mysterious places to study and explore.

On the other, they can be dark, difficult  to navigate, and even dangerous. Even our earliest ancestors  had a habit of exploring caves, long before we understood just how  geologically interesting they are. Every cave is unique, formed by slightly different geologic  processes in slightly different conditions.

And some of them really  belong in the record books. So here are five of our favorite  examples of record-breaking caves. Some caves are tiny, only a few meters long.

Others can be several kilometers. But the world’s longest known cave system is Mammoth Cave in Kentucky, USA. This isn’t just one long cavern, but a labyrinth of interconnected  tunnels comprising over 670 kilometers of documented passages.

That’s more than twice as long  as any other known cave system! Since the 1700s, people have been exploring  this cave for mining, tourism, and research, but archeological remains show that Native Americans were using  it as far back as 5,000 years. And yet, the history of the  cave itself is much, much older.

Like most caves on Earth,  Mammoth Cave is a solution cave, meaning all of those hundreds  of kilometers of tunnels were formed by the passage of water through rock. Here’s how it works: rainwater tends to dissolve carbon  dioxide from both the atmosphere and the soil it passes through, forming  a weak acid called carbonic acid. That makes the rainwater acidic, tain rocks and minerals, like limestone.

When rainwater hits the ground, it begins to flow downward  toward the local water level, sometimes flowing over the surface, and sometimes seeping through  cracks and gaps in the bedrock. Where flowing water meets limestone, it can gradually eat away at the rock,  widening cracks, deepening fissures, and creating caverns.   Given enough time, those  caverns can become massive. Geologists estimate Mammoth Cave began  forming around 10 to 15 million years ago.

Regions with lots of limestone or other  dissolvable rocks tend to be rich with caves, sinkholes, and other dissolution features. Places like this are called karst landscapes, including the area surrounding Mammoth Cave. So, the formation of a cave is  mainly dependent on two things: the local geology or the features of  the rocks, and the local hydrology: that is, the behavior of the water.

The water flowing through Mammoth Cave empties into the Green River. Over time, this river has  carved through the landscape, l lowering the water level, and the water in the cave followed, finding deeper passages  and carving deeper tunnels. These days, the tunnels of Mammoth Cave  are arranged into roughly five levels, with the oldest level of passages at the  top and successively younger levels below.

The lowest and newest tunnels are  flooded with water at river level. And on top of it all is a layer of sandstone, sitting above the highest levels of the cave,   protecting the vulnerable limestone from  wind and water erosion up at the surface. So, for millions of years, flowing water has carved out ever deeper and more complex passages, and a protective surface layer has  kept the older passages intact.

That’s how you get over 670 kilometers of cave. And yet, that’s not even all of it. The cave is still being  explored and mapped.

In 2021, explorers documented an additional  13 kilometers of tunnels. Keep in mind, that’s something people have to do. Each new section of cave  mapped means a group of people went down there with spelunking gear, measuring  tools, and navigation equipment.

So, Mammoth Cave is the longest cave we know of, and it just keeps getting longer! Mammoth Cave might be the longest known cave, but this next one has it beat for sheer  vastness. It’s the world’s largest cave, Hang Son Doong, located in Vietnam.

This main cave passage is  a mere 5 kilometers long, puny compared to the cave we just discussed, but some sections of Son Doong are an  incredible 150 meters wide and 200 meters tall. It’s been said that this cave is so big it could fit your average New York City block, or accommodate a 747 flying through. But while there aren’t skyscrapers or  airplanes in there, there is a jungle!

At two points in the cave,  the ceiling has collapsed, letting in sunlight, plants, and animals. These spots contain thriving patches of  jungle hundreds of meters below the surface. Once again, the formation of this incredible cave comes down to the nature of the rocks and the water.

On the geologic side of things, the rock layers surrounding the cave,  though susceptible to dissolving, are particularly sturdy, strong enough to  support large passages without collapsing for the most part. On the hydrologic side, there’s  just a whole lot of water. Son Doong sits in a tropical humid environment, and a massive river flows north  to south through the cave.   It’s estimated to have taken between 2 and 5 million years for  these huge passages to form.

Surprisingly, this giant cave was hard to find. Since it’s surrounded by  jungle, it’s pretty remote, and you need to get very  close to spot the openings. The first reported person to find the cave was  a local man who found it in the early 1990s, but it wasn’t located again until 2009.

Nowadays, Son Doong is a  well-known tourist attraction, so we probably won’t lose it in the jungle again. If you look up the world’s deepest  cave, you’ll come across two names: Krubera-Voronya Cave and Veryovkina Cave. Both are located in the Abkhazia  region near the Black Sea, and both are known to reach depths  over 2,000 meters below the surface.

Exactly which one is deeper is a bit tough to say. Both caves were discovered in the 1960s, and their maximum recorded depths keep increasing as spelunkers keep exploring them. As of this episode, the latest records  have Veryovkina Cave at 2,212 meters deep, and Krubera just a bit behind at 2,197 meters below their respective surface entrances.

Both caves are located in the Arabika  Massif, a high-elevation area which, once again, is a karst landscape full of caves. The formation of these super-deep caves is thought   to be the result of falling  water levels and rising land. The modern karst landscape seems to  have formed around 5 million years ago.

At that time, there was a major drop  in the water level of the Black Sea. Much like with Mammoth Cave, the water in the local landscape followed, s eeping and carving into the limestone  to find deeper paths to water level. Over the next few million years, tectonic forces uplifted the landscape, while at the same time creating  faults and fractures in the rock, providing deeper and deeper  passages for water to flow.

These days, the surface is quite high up. The entrance to Krubera cave, for example, is 2,256 meters above sea level, as the water flows down toward the Black  Sea and carves the whole way along. These caves are difficult to explore.

The passages are mostly vertical,  the temperature is cold, and it’s a long, long journey. The trip to the bottom of  Veryovkina cave takes three days. But humans aren’t the only ones interested.

Research reported in 2012 identified a variety   of invertebrates living below  2,000 meters within Krubera cave, making this the deepest known  underground animal community! Both caves are still actively explored Continued surveying is bound  to find deeper passages, and who knows what other surprises. Most caves are pretty young,  geologically speaking, usually only a few million years old.

It’s not that caves didn’t form earlier. But as time goes on, caverns tend to collapse, erode away, or become completely filled with sediment. Ancient infilled caves can be a great resource for geologists and paleontologists, but if you’re looking for a spelunking spot, you’re mostly stuck with caverns  that formed relatively recently.

But of course, there are exceptions. Recent research suggests that the  Jenolan Caves system in Australia has been open for over 300 million years. Jenolan Caves are Australia’s  most visited show caves, and like the others we’ve discussed, they  formed from water moving through limestone.

Although in this case, the dissolution process is thought to have involved  water falling from the sky as well as thermally heated  water rising up from below. During research into the  formation of Jenolan Caves, scientists collected and dated a variety  of sediments from different passages, reporting on them in a 2006 study. The oldest sediments in the study dated back to the Carboniferous Period, over 300 million years ago.

That’s well before dinosaurs even existed, back when trilobites swam the seas  and giant insects flew through some of the Earth’s first forests! And there’s no evidence that the sediment  was moved since it was laid down, suggesting some parts of this cave  have been open the entire time. If true, this would make Jenolan Caves  the oldest known caves big enough to be explored by humans.

The next question, of course, is  how an open cave could last so long. The researchers suspect the cave  originally formed near the surface, but was eventually buried under  new layers of rock and sediment, rendering the open caverns deep below the surface. Later, tectonic activity caused  the landscape to be uplifted, exposing the upper layers to wind  and water erosion until eventually, Jenolan Caves ended up just  under the surface again.

If this hypothesis is correct,  it makes you wonder if the caves are near the end of their long lifespan as erosion continues to do its thing. The history of these ancient caves,  like the others we’ve discussed, are fascinating geologic stories,  but they’re also informative. Understanding how caves form,  especially unusual ones, can help us figure out the best ways to  explore them, map them, and protect them.

The last caves on our list break a lot of records. They might be considered the  most difficult caves to explore, as well as the most inhospitable  and dangerous caves we know of. That’s because these caves… are on Mars!

That’s right, Earth isn’t  the only planet with caverns. Satellite images from the Mars  Reconnaissance Orbiter show that some parts of the Martian surface are dotted with dark areas that appear to  be holes in the planet’s surface. A survey using this data identified  over 1,000 of these holes, each one a potential cave entrance.

Unlike the other caves on this list, most of  these Mars caves were not formed by water, but by by a different fluid: lava. Geologists know quite a bit about lava  caves because they’re not unique to Mars. They form here on Earth as well, in  places like Kilauea Volcano in Hawai’i.

Here’s how these caves form. As hot lava streams away from a volcano, the center of the flow will  stay hot, but the outside, the parts where lava meets air, will cool. In some cases, the lava’s surface can solidify, creating a rock tunnel that still  has lava flowing through its center.

If all the lava eventually  drains away farther downhill, it can leave behind a tube of rock – a lava cave! The Martian caves probably formed the same way. The area of Mars with the  largest number of suspected caves is also home to three massive volcanoes.

Many of the holes are thought to be places where the ceilings of these caves collapsed. Not only do these features tell  us about Mars’s geologic history, they’re also potential areas for human habitation. Since the interior of these caves would  be mostly protected from the radiation and storms on the Martian  surface, some astronomers think they would be valuable refuges for  eventual human explorers to the red planet.

But it’s hard to identify caves from images  taken 400 kilometers above the surface. Some of these holes might not be caves at all, and some caves might have entrances  we can’t spot from the air. So, just like caves on Earth, we have a lot more to learn about Mars caves.

And, of course, these are just  the most distant caves we know of. It’s also likely that caves exist  elsewhere in our solar system, and maybe even beyond. But we’ll keep working on  the ones here at home, too.

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