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If you’ve got a question about the world, there’s no better tool than science for answering it. But sometimes, even science struggles to explain everything.

Some phenomena are so rare that scientists don’t get the chance to observe them in real time, while others are so common it’s hard to find one thing which explains every occurrence. And some are just such head-scratchers that scientists haven’t been able to figure out despite decades or even centuries of research. Let’s take a look at seven of the coolest, weirdest mysteries that scientists are still trying to solve.

First up is something so common you might have seen it yourself: ball lightning. Surveys show about 5% of people have seen these strange, glowing orbs of light and records of them date back to antiquity. They have the appearance of lightning, but instead of a quick flash, each ball can last from seconds to minutes.

Most are no bigger than your head, but the largest can be more than a meter across! Some observers have even reported the orbs entering buildings through windows or traveling along the ground. Outside, ball lightning seems to appear alongside thunderstorms, but during World War II, it was often observed by pilots and even in the engine rooms of submarines.

Over the years, scientists have proposed lots of different ideas to explain what’s going on. One idea is that when lightning strikes during a thunderstorm, it vaporizes some of the silicon found in everyday soil and releases it into the air. As the vapor condenses into flecks of silicon dust, it picks up an electric charge and clumps together into loose “balls.” Individual silicon molecules then combine with oxygen from the air to form silicon dioxide, which releases the energy that causes the ball to glow like lightning.

And there’s some support for this idea, as researchers have succeeded in creating smaller, shorter-lived glowing balls in a lab by vaporizing silicon. But lightning doesn’t strike underwater, let alone in a submarine. Another possible explanation for ball lightning is that the electrical energy in a storm creates microwaves and that, just like in your microwave oven, those waves energize water molecules in the air.

Through a process called stimulated emission, air molecules shed that extra energy as a glowing light we see as ball lightning. Stimulated emission might also help explain the observations made inside planes and subs. Those closed environments could trap the energy and enhance the process.

It’s also possible that what looks the same in those places is actually a different phenomenon than what we see outside. Scientists will have to be in the right place at the right time to study actual cases to figure out what’s going on. Ball lightning might look like lightning without thunder, but what about the opposite: thunder without lightning?

That’s what some in the US call a skyquake. In Japan, their name translates to “rumbling of the sea,” and in Bangladesh, it’s “Barisal guns.” Whatever the name, what people hear is a loud, thunder-like boom coming from a perfectly-clear sky. They’ve been reported for centuries across an incredible array of circumstances, which could mean that a bunch of different processes are all producing similar results.

In the modern world, some are probably the shockwave produced when a far-off jet goes supersonic. Of course, that doesn’t explain skyquakes from before the invention of jets and, even today, supersonic flight is banned around most places people live. Some have suggested small meteors could be the source of far-off sonic booms instead.

Since the rocks burn up quickly, they could be visually gone before the noise reaches the ground, and they’d be hard to spot anyway during the daytime. Other scientists think that rumbles heard near a coast might come from giant bubbles of methane released from the ocean floor. The gas molecules are physically trapped within crystallized water, forming substances called clathrates.

Occasionally, the strength of these clathrates can fail catastrophically, releasing a bunch of methane all at once. When it reaches the surface, all that gas can create a shockwave which makes a loud boom, and sometimes, even launches a tsunami. Far from the sea, they could be related to their better-known cousin, the earthquake.

Shallow, weak earthquakes may not shake the ground enough for us to notice, but the sound they produce could still be loud enough to hear. So next time you hear a rumble in the sky, it could actually be the ground. Or the sea.

Or a plane? Alright, let’s move on. Skyquakes and ball lightning have remained unsolved for centuries, but there are more recent discoveries so baffling that they earned a spot on this list.

Since 2007, astronomers have detected a series of high-energy pulses coming from seemingly-random directions in space. They’re called fast radio bursts or FRBs because each lasts just a few milliseconds. Here’s the really weird part: all but one have never been heard from again.

So it’s probably not ET trying to get in touch. Astronomers have figured out that whatever’s producing them is way outside our galaxy, billions of lightyears from Earth. For them to appear so bright means the source must be astonishingly powerful.

Our only hint about what’s going on comes from the one source that’s ever repeated, and studies suggest it may be one of the universe’s most tortured objects. Many experts think it’s a neutron star, the dead core of an exploded giant star. And not just any neutron star, but one that’s having a really bad day.

One hypothesis suggests it’s in the midst of being devoured by a nearby black hole. Another claims it’s caught in the supernova blast of yet another dying star. Either way, all this trouble is causing it to emit a series of fast radio bursts.

The thing is, since no other FRB repeats, we don’t know if this tells us anything about what’s causing them. Then again, maybe the other sources do repeat, just over a much longer timescale. A decade is pretty short on the cosmic scale, after all!

Another “cosmic” mystery probably isn’t coming from space at all! For centuries, people have reported finding star jelly on the ground or in tree branches, especially after a meteor shower. This gooey material seems otherworldly, but one thing we can be sure of is that it’s not coming from outer space.

Something as delicate as star jelly just wouldn’t survive the heat and pressure of entering Earth’s atmosphere. It is a good example of how things happening close together can lead scientists astray. Most likely, long ago, some observers noticed star jelly around the time they saw a meteor shower, and the connection just kind of stuck.

But if it’s not goop from the galaxy, what is it? One clue might come from where it’s often found, near bodies of water. Star jelly bears a striking resemblance to the spawning material that contains eggs of reproducing frogs.

So in some cases, it might be frogspawn or frog ovaries eaten, and then thrown back up, by another animal. We’re a long way from black holes destroying neutron stars here. There are also no shortage of strange gel-forming creatures, from slime molds and fungi to bacteria and blue-green algae.

And at least one case was determined to be simply chemical in nature, a purple gel found in Texas in 1979 turned out to be a cleaning agent from a nearby battery factory. So it may just be that there are way too many strange gelatinous things in this world to attribute star jelly to just one source. Determining the jelly-in-question every time would mean taking careful samples, finding experts, and maybe even running DNA tests or something.

That’s a lot of time and money for scientists to spend just to answer “what is that goo?” Bacteria are potential culprits in another unexplained natural phenomenon: forest rings. These circular patterns are made by trees whose growth has been stunted compared to the rest of the forest. They can range in size from a few meters to a few kilometers and are pretty much only visible from the air.

Around 2,000 are known in the world, but scientists estimate that that might just be a quarter of the true number. And unlike crop circles, forest rings aren’t created by people. Although, it’s pretty funny to imagine someone slowly poisoning trees for decades in the middle of nowhere.

Talk about a long con! In the past, scientists thought they were the result of a harmful fungus spreading out from a single tree. But sampling didn’t reveal any clear culprits.

More recent ideas have focused not on the trees themselves, but what’s below them. Soil samples taken in some rings reveal an unusually-high concentration of methane. Other measurements showed the potential for underground metal deposits.

Either one can create an excess of negative electric charge and basically turn the ground into one big natural battery. That acidifies the soil and harms tree growth. Bacteria may play an important role, especially when it comes to the methane.

Microbes far underground might produce the stuff, while those closer to the surface can consume it as part of the reaction. But that’s all mostly speculation at this point. Geologists and biologists have only recently started digging into the mystery, and are now trying to set up their own artificial forest rings to try to find out what’s really going on.

Unseen geology is also a suspect for one of the world’s weirdest light displays. For decades, locals near the Hessdalen valley of Norway have reported a series of strange, floating lights. These so-called Hessdalen lights can be as large as a car and float in place for hours.

In the 1980s, they were seen as often as 20 times a week. If you ever see one, don’t touch it! Individual lights have been measured to emit as much as 19 kilowatts of power!

Because the Hessdalen lights always appear in the same area, scientists have been able to make better measurements of them than many other unexplained phenomena. Even so, there are still a wide range of ideas about what’s going on. One is that we’re seeing the effects of a Coulomb crystal, which honestly just sounds like something from science fiction.

Coulomb crystals can form in plasma, which is basically a form of electrically-charged gas. They’re not physical in the sense that you could pick one up. Think instead of a kind of force-field cage within the gas.

If enough charged particles were trapped inside such a crystal, their combined glow could shine brightly. As for why Norway, of all places, some scientists think there is a unique situation where a source of atmospheric dust comes into contact with a natural reservoir of radon gas. Radon is radioactive and, as it decays, it emits radiation that could electrically charge the dust in a way that makes Coulomb crystals more likely.

Other researchers think the lights are actually a special case of something we’ve already mentioned: ball lightning! Instead of a storm, though, the lightning would be thanks to this region’s unique geology. Rocks in the valley contain a lot of quartz, which generates an electric field when strained under physical forces like compression.

All that excess electricity could launch a lightning ball over and over in the same area, resulting in a beautiful, but dangerous light display. One thing that’s pretty clear about all these mysteries is that aliens are not the answer. Except for maybe this last one, well, from a certain point of view.

The biggest unsolved mystery in geology isn’t lights in Norway, but the outside of rocks all over the world. Wherever there’s arid conditions, geologists find rocks covered in desert varnish, a dark substance just a few microns thick. Petroglyphs, the rock art of some ancient cultures, were often created by scraping away the varnish to reveal the brighter rock below.

One particularly intriguing observation is that, although the composition of desert varnish varies, it doesn’t seem to relate to what makes up the rock underneath. And scientists can generally detect microbes in the stuff, but that doesn’t mean that they’re the source of it. Some elements found in the varnish, like manganese, are common microbial waste products, but the real question is whether life is producing the varnish, or simply there, too.

And there is another, much more speculative idea which is kind of fun. Some have proposed that desert varnish might be the most visible sign of the so-called shadow biosphere. That’s the idea that there’s a whole world of undiscovered microscopic life on Earth that we haven’t noticed because it doesn’t work how we expect.

Imagine life with a totally different chemistry. Would we recognize it? It’d almost be like alien life right here on our own planet.

This is an idea rooted more in philosophy than science. But it is an important warning as we begin to look for life beyond Earth. The universe may be full of creatures, but if they work fundamentally different than anything we know, how will we find them?

In a way, that’s the value of these unsolved mysteries. They force scientists to consider our own, well-studied world in new and surprising ways. Each question might be small, but the investigation has the chance to reveal big new answers.

Thanks for watching this episode of SciShow! If you liked learning about the science behind these weird phenomena, you might like our episode on three times science debunked the paranormal. [♪ OUTRO].