This episode is sponsored by 80,000 Hours. 80,000 Hours is a nonprofit that aims to help people have a positive impact with their career.

Head to 80000hours.org/scishow to start planning a career that can help solve one of the world’s most pressing problems. ♩ INTRO ] If you look at the East Coast of the United States from way, way high up, you’ll see a bunch of really diverse geological features. And as impactful and lovely as many of them are, most of them generally make sense.

Like rivers. We know rivers. Mountains, yeah, you generally kind of get mountains.

But how about the hundreds of thousands of oval-shaped swamps littered across nearly a dozen Eastern states? Because that’s a thing. Scientists have been puzzling over them for a long time, trying to figure out where they come from.

And recently, they’ve started closing in on an answer. These features are known as Carolina bays. They’re isolated, shallow depressions that range from southern New Jersey to northern Florida, the Carolinas.

They’re about 1 to 3 meters deep, and they go from about the size of an American football field to around 12 kilometers across. But now, many of them have been paved over or plowed through, so we don’t know exactly how many exist, but there could be as many as half a million. And these bays have a few defining features.

Many of them have rims of sand along their south and east edges. And they all seem to have a similar elliptical shape that generally points from the northwest to southeast. They’re also really cool ecologically.

Since they’re not connected to rivers or other channels of water, Carolina bays usually become isolated wetlands fed by rain and groundwater. Many of them go through seasonal wet and dry periods. As a result, the animals that live there, like turtles, deer, raccoons, and opossums, are adapted to changing water levels.

Many endangered species, like wood storks, flatwoods salamanders, and gopher frogs, also call these bays home. Scientists have spent plenty of time studying these fascinating ecosystems over the years, but still, no one has been able to pin down exactly how these places formed to begin with. One idea is that they formed when a meteor broke up and showered the earth with its debris.

After all, the round depressions do look a little like craters. But the bays are mostly concentrated on sandy soil, whereas you’d expect debris from a meteorite to spread out evenly over all different kinds of soil. So, scientists have moved away from that hypothesis.

Other ideas suggest that the bays are the remains of sinkholes or that they were made by schools of fish back when this whole area was underwater. But as of now, the strongest hypothesis is that they’re the remnants of ancient, chilly lakes. Scientists drew this conclusion in part thanks to a technique called optically stimulated luminescence.

This is a cool technique that can be used to date geological events, by revealing the last time a given quartz crystal was exposed to light. Essentially, when quartz is buried, electrons within it accumulate in little imperfections in the atomic lattice. Then, when the quartz gets exposed to light, the incoming energy basically jostles these electrons out of the traps and sets them free.

So, you can kind of think of a quartz crystal as a stopwatch In the lab, scientists can measure how many electrons escape a quartz crystal when it’s exposed to light, and that lets them estimate how long it’s been buried. Now, the bottoms of these sandy Carolina bays are full of quartz crystals. And, according to this technique , the bays likely formed about 92 to 95,000 years ago, during the last ice age.

That’s when these crystals last saw the light. Back then, this part of North America looked completely different. The air was colder, winds were stronger, and there was less vegetation on the landscape.

It would actually have looked kind of like parts of Alaska today. Which… also happens to have some weird, oval-shaped pits. In Alaska, these are known as thermokarst lakes.

They initially form in areas where the ground repeatedly freezes and thaws until it collapses, kind of like a giant pothole. Then, once there’s a pit to work with, consistent, high winds blowing mainly from one direction begin to shape these lakes into ovals that point in the direction of the wind. At the same time, wind-blown sand also begins to accumulate along the rims of the lake, especially on the downwind sides.

Now, it just so happens that, some 90 thousand years ago, the East Coast would have had winds similar to these, blowing in from the northwest. Which has led scientists to suggest that Carolina bays are actually ancient thermokarst lakes! This hypothesis potentially explains the consistent oval shape and rims of sand that characterize these formations.

It also could explain why they’re generally aligned in the same direction. If researchers are right, these pockmarks might be the scars of a much colder, drier, and tougher East Coast from the distant past. But today, these scars have evolved into hundreds of thousands of homes for a wide variety of life.

Thanks for watching today’s episode of SciShow, which was brought to you by 80,000 Hours. 80,000 Hours is a nonprofit that aims to help people have a positive impact with their career. Your career is on average 80,000 hours long, so it’s a pretty big opportunity to make a difference. If you’re in a position to use your job to make a positive mark on the world, but you’re not sure how, they want to help.

They provide resources including guides on how to have a high-impact career, a curated job board with career opportunities they think are especially high-impact, and even a podcast, with interviews from experts who already have careers in these areas. I especially like the podcast episode where they talk honestly about navigating a career with depression, anxiety, and ADHD, and how to make an impact when you have a mental illness. And all their resources are free all they want is to help you make a difference.

Click our link in the description or go to 80000hours.org/scishow to be sent a free copy of their in-depth career guide, as well as to sign up for their newsletter to get updates on their research and new job opportunities. ♩ OUTRO ]