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Lots of bodies of water, like rivers and ponds, they change shape and size over time. But lakes?

Like, we usually think of lakes as being pretty permanent. Yeah, of course, water levels will rise and drop during years of heavy rain or drought, but lakes themselves, they don't go anywhere, right? Well, except I wouldn't be talking about it if they didn't; some do, and around the world, there are lakes that disappear without warning, and then, even stranger, they come back.

This can happen for lots of different reasons, and understanding these mysterious lakes can show us surprising things about Earth's geology, and how water is often connected to hidden processes under the surface. One of these disappearing acts happens every year in the Cascade Mountains, in one of Oregon's so-called “Lost Lakes.” If you saw this lake in the winter, you wouldn't notice anything strange. During the wet winter months, it fills up with rainwater and runoff, and covers over 300,000 square meters of land.

But when the rainy season ends in the spring, things get weird. Around that time, the lake starts to shrink, and it keeps shrinking until, by summer, the whole thing is a giant meadow. Now, you might be thinking that this lake just evaporated, right?

But a few months isn't enough time for a huge lake to just evaporate into thin air. Especially because, let's be real, Oregon isn't that hot and dry in the summer. Instead, something else is happening; something way cooler.

The lake is actually leaking constantly through a hole in the lakebed that's around 2 meters wide. It's basically like having a bathtub drain that's constantly open. You just don't notice it in the winter because there's enough precipitation to fill the lake faster than it can drain away.

Geologists suspect that this hole, which is not very big, isn't just some random opening in the ground. Instead, they think it leads to a collapsed lava tube. Lava tubes are natural tunnels that form as lava flows down the slopes of a volcano.

The lava's outer layer cools into a hard crust, but the inner layers are still molten and they continue to flow downhill. That drains the tube and leaves behind a hollow channel. The lava tube under the Lost Lake formed over 12,000 years ago, and it carries water from the lake into the McKenzie River.

From there, it ends up more than 9 kilometers from where it started, in Oregon's Clear Lake. Besides being super cool, this shows us that bodies of water aren't just connected by the usual avenues, like rivers and streams. Ancient volcanic tunnels are still shaping the way water travels today.

Next, in the Patagonia region of Chile, another disappearing lake performs its vanishing act on a much less predictable schedule. Lago Cachet Dos is a glacial lake that fills with melting ice from the Colonia Glacier. Normally, this glacier acts as a dam to keep the lake in place.

But if the lake gets too full, things reach a breaking point. As the lake fills and water pressure builds, water can start wearing through the ice. And eventually, the water bores through a weak area at the base of the glacier.

That creates a channel that extends eight kilometers underground until the water breaks through the other side of the glacier. And then the whole lake comes rushing out that tunnel! An event like this is called a glacial lake outburst flood, or a GLOF.

Back in 2012, a GLOF drained a whopping 200 million cubic meters of water in less than a day! That tripled the volume of the river it flowed into. Luckily there aren't too many people in the area around it, but the flooded river did do some damage to roads and farm animals.

As the glacier moves, it collapses that underground tunnel, and the lake fills back up, so the same process can happen again and again. Lago Cachet Dos has drained over twenty times in the last decade. And as climate change speeds up melting, the number of GLOFs is likely to increase.

GLOFs don't just happen in Chile, though. These glacial lakes appear, and disappear, in other mountain regions, like the Himalayas, the Alps, and even the Rockies. So understanding how they and their outbursts are connected to climate conditions and other waterways can help people stay safe from unexpected floods in the regions around them.

Finally, let's bring it back to my home state: Florida. Where there's a third lake in Tallahassee that's famous for yet another type of dramatic disappearance. It's called Lake Jackson, and it's pretty flat and shallow; honestly, it's pretty boring at first glance.

But if you look underneath the surface, you'll see something interesting: sinkholes. Lake Jackson has two sinkholes that drop down more than 15 meters towards the Floridan Aquifer. They're often plugged up with sediment, which allows the lake to fill up.

But it's a fragile arrangement. Every once in a while, Lake Jackson gets enough water through rain or runoff to break through those plugs and plunge through the sinkholes into the aquifer. When that happens, the lake can empty in as little as a day, draining up to 37,000 liters a minute.

Then, over time, sediment naturally plugs the sinkholes again, and the lake fills up. This has happened at least a dozen times since 1840, and, as catastrophic as it sounds, this rush of water is actually a good thing for the local ecosystem. The draining kills off invasive plants and even flushes out native ones, which can start to take up too much space, too.

During these dry periods, people can also go in and remove pollutants that drain from urban areas and settle into a thick layer of ‘muck' on the bottom of the lake. So this unusual cycle of draining and refilling actually plays a pretty important role in Lake Jackson's ecosystem. Whether these lakes calmly come and go with the seasons or drain suddenly in dramatic and unpredictable events, the fact that they vanish and reappear reveals surprising connections.

Our waterways are joined by things as wide-ranging as ancient volcanic activity and recurring sinkholes, which all contribute to how water moves around today. Thanks for watching this episode of SciShow! And thank you to our patrons!

It takes a lot of people to make a SciShow video, and we couldn't do it without your support. And if you're interested in supporting us, you can head over to patreon.com/scishow to learn more. [♪ OUTRO].