This episode is brought to you by Nature’s Fynd, a fungi-based food company for optimists.

Click the link in the description to discover the science behind Nature’s Fynd fungi-based foods. [♪ INTRO]. Geologists love to look at strange rock formations and figure out exactly how they got there.

Usually, this is fairly straightforward. Combine a volcanic eruption here with a little bit of erosion there, and boom, you’ve got yourself a really striking cliff! But not all the features or behaviors of this planet are so easy to figure out.

Some have kept geologists scratching their heads for decades or even centuries. And there are some mysteries that even modern scientists haven not solved, yet. In terms of high-priority geological mysteries, the ground randomly exploding seems like it should be near the top of the list!

And that’s exactly what’s happening in parts of Siberia. Massive craters have been appearing in the permafrost, around 20 of them so far! They’re up to 90 meters in diameter and 70 meters deep, the aftermath of colossal explosions.

Now, few people have witnessed the actual exploding, and never from close up, but one village saw 5-meter flames that burned for an hour and a half! Now, geologists do know that these are from some kind of explosive gas building up under pressure, mostly likely methane. Gas samples from the bottom of one crater were nearly 10% methane.

But no one’s exactly sure where the gas is coming from, how pockets of it are forming, or where the next one will be. One explanation is this is all the result of melting methane hydrates, which form when methane gas is incorporated into the crystal structure of ice. The methane itself is thought to come from deeper within the Earth.

You see, when organic material buried long ago is subjected to extreme heat and pressure underground, it can produce methane which rises upwards to the frozen permafrost, where it can get trapped in ice. And if this is what’s happening, then more explosions are probably on the way. With climate change warming the Arctic, methane hydrates are melting faster!

But researchers are still debating the details here. Other studies blame microbes for the methane. And some don’t think it’s really about methane at all. one study points to highly pressurized carbon dioxide as the cause of the explosions.

And this is all critical to figure out, and, ya know, fairly soon, because satellites have found 7000 hills that might be ticking time bombs. Until we know what’s actually happening, we will not know how many of these are dangerous. What we do know is that these hills form quickly, over a few years.

And studying them directly would probably shed some light on things. But they can be under very high pressure, so that’s kind of tough. It’s like when something is maybe about to explode, a researcher doesn’t want to go out there and poke it to take a sample.

Because what if you are the next crater? Explosions and impacts are two of the very few things that create near-perfect circles in the Earth, so circular features tend to be worth investigating. Well, it turns out you’ve been looking part of a near-perfect circle, at least by Earth’s standards, every time you look at a map of the world.

And once you see this, you will not be able to unsee it. It’s called the Nastapoka Arc. It’s 650 kilometers long, and makes up a big section of the Hudson Bay in Canada.

Clearly, something big happened here. The first idea was that this is an impact crater, the site of a massive thing from space hitting the Earth. There are plenty of other circles that we know formed this way, like nearby Lake Manicouagan....

Another circle on maps that you just won’t be able to unsee! But when something hits the Earth, it leaves behind a number of pretty telltale clues. Things like weird rocks thanks to shock metamorphism, when the heat and pressure from a sudden impact deforms and even alters the minerals in rocks.

Or, shatter cones, unique patterns of striations on rocks that only form beneath impacts. And the Nastapoka Arc doesn’t have either of those! That’s left scientists with one other hypothesis: that the curve was once the boundary between two tectonic plates.

Essentially, close to 2 billion years ago, two continents may have collided and closed an ancient ocean. And as the plates collided, one was thrust up on top of the other, flexing it into a semi-circle. But circular boundaries between plates aren't common.

The idea is that here, as material from one piled up on the other, an unusually massive amount of rock built up in a relatively small area, the center of the circle we see today. This weight of all of that was so much that it pushed the lower plate down in a single spot, creating a semi-circular bend in the crust. Now while that could make sense, other areas where there have been similar plate collisions are not perfect circles.

So some geologists still think that this was an impact, perhaps a really ancient one, so the usual evidence has been lost to time. We’re left waiting for some adventurous geologist to find either clear evidence of an impact or a unique plate collision to close the loop on this very large, very mysterious circle. Not every mystery can be seen with the naked eye.

A patch of the Indian Ocean is missing gravity, and no one’s entirely sure why. Now, it’s not that you can, like, go here and float. You couldn’t even jump any higher if you were standing on a ship there.

These are tiny differences that geophysicists detect with very precise instruments. Studying Earth’s gravity at different places allows us to see beneath the surface of our planet. See, higher gravity means that something is more dense down there, and vice versa for lower gravity.

Except that, earth scientists are totally stumped by the largest gravity anomaly of them all, a massive spot of low gravity in the middle of the Indian Ocean. It could be that the mantle isn’t the same in every spot, and there's a plume of hotter, less dense material down there. But some think it goes all the way down through the planet’s mantle, essentially suggesting there’s a, a dent in the mantle!

The idea is that since the outer core is liquid, the boundary between it and the mantle can undulate slowly over time, kind of like it has waves. So the anomaly is the trough of one of those waves. Others think there are slabs of old continents sitting down there that plunged under the crust at the bottom of the ocean around the time of the dinosaurs.

Researchers are still learning about what happens to tectonic plates after they’ve sunk beneath the crust. But, it turns out, they can take hundreds of millions of years to mix back into the mantle. And the hypothesis here is that these subducting slabs bring water down with them.

The water can reduce the melting temperature of the rock around it to create buoyant plumes of less-dense magma, and those could be responsible for the low gravity at the surface. There’s certainly no shortage of ideas. But determining which one is the right one will require some higher-quality images of what’s going on beneath the seafloor.

And that’s why geophysicists have stuck a bunch of seismometers in the middle of the ocean! They’re hoping the way seismic waves from earthquakes interact with whatever is down there will give them clues to what’s really going on! Our next mystery takes us to the small island of Anjouan, off the northern coast of Madagascar.

See, this island has the wrong type of rock on it. And by that, I mean it is, it is an unexpected type of rock. You see, Anjouan is a volcanic island, but on its surface, you can find a rock called quartzite, which, frankly, it just should not be there!

It’s pretty clear how the island itself formed: an undersea hot spot volcano slowly built up rock until it made an island, similar to the islands of Hawaii. And this one is relatively young by geologic standards, around 4 million years old. And all this makes Anjouan the last place you would expect to find quartzite.

Now that requires some explanation, but not a lot of explanation because quartzite begins as a quartz-rich sedimentary rock like sandstone, and that is formed by quartz grains eroding from a continent and then slowly accumulating at the end of a river. The sandstone is then buried and fused into quartzite under heat and high pressure. And that’s the mystery here, the quartz shouldn’t have eroded from the volcanic rock of the island itself!

It must have come from some larger continent. And besides all of that, the quartzite is likely much older than the island itself. They’ve been dubbed the “impossible rocks” by the researchers studying them, and their history is still very much an unanswered question.

The current hypothesis is that they came from the African continent, possibly when Madagascar broke away, but it’s not clear how this would have happened. So the next step for the researchers is to figure out exactly how old the quartzite is and compare it to the age of rocks in East Africa and Madagascar to see if they find a match! One of the most long-standing geologic mysteries is also one of the most seemingly mundane.

Mima mounds might not look like much at first, they’re just… mounds of sediment, most famously found in Washington state, though similar formations exist on every continent except Antarctica. Each can be between 2.5 and 15 meters across and up to 3 meters high. So, like, they’re just not like, super impressive features?

Still, these little bumps have been haunting the dreams of geologists for almost 200 years! We simply don’t know how they form! There have been over 50 explanations proposed.

Everything from earthquakes shaking the sediment to winds building them during droughts to insects slowly constructing them bit by bit. In 1942, scientists proposed that pocket gophers were responsible!... and believe it or not that is still the leading theory to this day! A 1987 study added small bits of iron to the mounds and tracked them using a metal detector, and that showed that, when the gophers dug a burrow, they pushed the removed material, including the metal bits, uphill!

Then, in 2013, a computer model took those results and extrapolated them over hundreds of years. And, at least according to the math, these wee gophers could build that kind of mound… eventually. The process is predicted to take hundreds of years, an amazing multi-generational project for the little rodents!

We still don’t know why the gophers would do this, though. After all, it takes more energy to push sediment uphill when you’re digging a burrow. Also, a model is far from conclusive proof, so other ideas could end up being right.

So what we really need is for someone to take two flat areas of land, stick some gophers in one and keep gophers out of the other, and then see what happens over about 500 hundred years…. You know, just some simple, straightforward science that we’d all be long dead before we got the results of! Our final mystery is that every 26 seconds, like clockwork, the Earth rumbles.

Seismometers first heard this strange faint pulse, called a microseism, in 1961. Since then, it has been detected all over the world, except, strangely, not in South America. By comparing the signals at different seismic stations, scientists have triangulated the signal’s origin.

This leads to two spots. The strongest signal from the 1961 pulse was attributed to the Gulf of Guinea, in the Atlantic Ocean, off the coast of Nigeria. The second is in the western Pacific Ocean near Fiji, but that is on the other side of the earth.

It’s actually exactly on the opposite side of the world from that first spot! So scientists think it could actually be originating in the Gulf of Guinea, and then the seismic waves are traveling across the entire planet and concentrating on the other side! But no one is sure what that ultimate source is or why it occurs at such regular 26-second intervals.

One study suggested it could be related to some part of the volcanic process deep beneath the surface. But that wouldn’t explain why it is stronger during storms, and overall stronger when it’s winter in the southern hemisphere. Instead, those both suggest it has something to do with waves in the ocean.

Some experts hypothesize it’s simply that the African continental shelf is shaped in such a way that when waves hit it, they rumble through the Earth. A similar, though less impressive rumble coming from the seas around Sicily was tracked down to ocean waves by comparing seismometers to wave data from buoys in the ocean. So that kind of set-up could perhaps shed some light on this 26 second pulse.

But the Italian rumble was conveniently close to Mount Etna. That meant researchers were able to re-purpose an array of seismometers that was already there to monitor the volcano. Nothing like that exists in the Gulf of Guinea area now, so getting the data needed to support the wave theory won’t be so easy.

Still, regardless of what’s causing them, the microseisms have come in handy: they’re so regular that they’ve been used to sync up the clocks of seismometers around the world! And there you have it! Just six of the many mysteries of this planet that we have yet to solve.

Despite having studied Earth for thousands of years, we still have much to learn! It’s kind of the same way with agriculture. Yes, we’ve been at it for millennia, but it turns out that new discoveries can still change everything.

Just ask the scientists at Nature’s Fynd! They’re taking this fungi, first discovered through NASA-funded research, and growing Fy Protein™, which is much better for the environment than meat! Because, let’s face it, we don’t exactly have room on the planet for more livestock.

Their amazing fungi protein is packed with nutrition, and there are no animals required! You can learn all about the process for yourself at the link in the description. [♪ OUTRO].