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Phong Nha-Kẻ Bàng National Park in Vietnam has some of the most amazing caves in the world.
The park is a stunning example of karst topography,
where water has carved the limestone bedrock into caves and underground rivers.
Any of those caves is worth a visit, but many people are interested in one in particular:
Hang Sơn Đoòng.
This is a cave of extremes: It’s the largest by volume; has a huge, fierce river; and has
some really unique features that surprised even the experts who have explored there.
For one, it has two jungles!
Inside the cave are two patches of full-fledged jungle, with ferns, flowers, animals, and
full-grown trees.
One patch is called Watch Out for Dinosaurs, and the other is called The Garden of Edam.
These jungles are able to grow in a cave for two main reasons.
One is that there’s enough vertical room for trees to grow.
Water has worn away so much limestone that the ceilings in these areas are over a hundred meters high.
But also, these jungles sit beneath holes in the cave ceiling, called dolines.
They let in sunlight and rain, and they also create a passage for animals to get in.
So, birds, bats, and monkeys can get in and out to visit or pollinate the plants.
The jungles aren’t just a weird curiosity, though: They likely have an impact on the
conditions in the rest of the cave.
Which leads us to the second unusual thing about this place: its weather.
Technically, all caves with air in them have weather systems.
Because that’s basically what weather is: air doing stuff.
But Sơn Đoòng’s size, as well as the dolines and jungles, make it an extra complicated system.
Admittedly, there aren’t a ton of papers exploring the cave’s weather.
But based on what scientists know about how weather works in general, there are things
we can work out.
Like why the cave is super windy.
Wind is driven by pressure and temperature gradients.
If you have an area with high air pressure or high air temperature, that air will move
to an area of lower pressure or lower temperatures.
And in Sơn Đoòng, you have a bunch of gradients.
Like, in the deep parts of the cave we know about — which are just shy of 500 meters
underground — the air is warmer than the air near the surface.
And because Sơn Đoòng has such high ceilings, there’s room for these temperature differences
to drive convection currents.
On top of that, those dolines letting in sunlight can also create areas of local warmth during
the day, as well as places for air to escape, further altering the cave’s air dynamics.
Couple that with the daily cycling of carbon dioxide and water vapor around the jungles
from the plants, which alters the local air pressure, and you can get some strong winds.
Like we mentioned earlier, we’re still learning how these systems work
But in other caves, strong winds can help bring microbes, plants, and animals into cave
systems, and they can contribute to erosion.
So it’s worth learning more.
Finally, Sơn Đoòng also has some unique geology stuff going on.
Specifically, its cave pearls are unlike any others in the world.
Cave pearls are spheres of the mineral calcite that can form in limestone caves.
And unlike other calcite structures — like stalactites — they’re not attached to anything.
Overall, these things tend to be, well, pearl-sized.
Big ones might be a couple centimeters across.
But in Sơn Đoòng, there are pearls the size of baseballs.
And we’re not sure why.
The answer depends on how these pearls formed, and there are a few ways that can happen.
Some form in warm, shallow pools full of dissolved calcite, when a bit of sediment gets suspended
in the water and calcite precipitates onto it.
Pictures of Sơn Đoòng’s cave pearls show them resting behind rimstones, which are structures
that once formed the boundaries of shallow pools.
So that could be how these pearls got there.
Of course, most water in a limestone cave is warm and highly saturated with calcite.
And most caves don’t have baseball-sized cave pearls.
But the water in Sơn Đoòng could have been extra warm and saturated.
And also, those pool systems may have existed for an exceptionally long time before drying
out, which would have given the pearls more time to grow.
There is another idea, though.
Because cave pearls can also form when calcite-rich water drips down from the ceiling.
When a water droplet falls, it has mass and velocity — that is, momentum.
And when the droplet hits the ground, some of that momentum is imparted into the sediments it hit.
If that momentum is enough to lift up those sediments, those sandy particles can act as
a core for the calcite in the water to coat.
And as long as the water impact can lift up that growing sphere, this process can repeat,
and you can grow a cave pearl.
If you’re a water droplet in a cave with a low ceiling, you don’t have enough distance
to accelerate to a high velocity — you’ll hit the ground with less momentum.
But dropping from a high ceiling lets you achieve high momentum, and bigger cave pearls.
And the ceilings in Sơn Đoòng are hundreds of meters high!
So that could also explain the size of the cave pearls!
Until more research is done in the cave, we won’t know for sure.
But as that research is done, we’ll get a better picture of the cave’s geology,
of its weather, and of all the species living in its jungles!
Now, studying caves is about more than just jumping into a hole with a headlamp.
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