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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
owering 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 accomodate 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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