For decades, geologists were baffled by massive underground tunnels in Brazil — but the mystery has finally been solved. Scientists have confirmed these enormous structures, known as “paleotunnels,” were carved by giant prehistoric sloths living between 8 million and 10,000 years ago.
What do these tunnels reveal about Earth’s past megafauna? Could there be more undiscovered beneath our feet? Dive deep into this groundbreaking discovery and learn why these extinct creatures were nature’s original diggers.
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00:00Deep inside the heart of the Amazon rainforest lies a cave so massive, it defies logic.
00:08Stretching for 2,000 feet, this tunnel is so big a human could easily walk around inside of it.
00:15Its walls are etched with deep, mysterious claw marks.
00:18But this is just one of many.
00:21If you travel to the south of Brazil, you'll stumble upon over 1,500 similar caves.
00:27Ranging from modest, tiny burrows to massive passageways.
00:32These are not your average spooky, bat-filled caverns.
00:36They're what researchers call paleoburrows, also known as ancient underground structures dug by creatures so colossal, they make modern elephants look small.
00:48Whatever creature dug those tunnels displaced 800 elephants' worth of dirt.
00:52I know, I know, it still looks like a normal cave, but experts noticed this one was special because the walls were too neat and rounded.
01:01And the floor was too smooth.
01:04Besides, no water or erosion leaves such signs of serious digging power.
01:09These Brazilian caves are the largest geological records of animal activity on the planet.
01:14It's like stumbling into a prehistoric house made by creatures the size of cars.
01:21To unlock this mystery, researchers analyzed the scratch marks on the walls and realized they had probably been made by giant ground sloths.
01:30Despite being huge, these animals were not earthbenders, they had to work in groups if they wanted to dig such long tunnels.
01:48This is why scientists believe many sloths shared the same tunnel and dug it together.
01:54Even some giant armadillos may have helped with the digging.
01:58After all, they were ridiculously skilled excavators and lived there too.
02:04Okay, but why would a giant sloth or armadillo need a 2,000-foot-long underground lair?
02:10Great question.
02:12And the truth is, scientists aren't totally sure.
02:15It doesn't make sense for them to dig tunnels this massive just to escape predators.
02:21Especially since the tunnels were big enough for a saber-toothed tiger to crawl in, for example.
02:27So, unless they installed prehistoric security systems, this theory has holes.
02:32Ha ha ha.
02:33Pun intended.
02:34The best bet is probably climate.
02:37Back then, everything was drier and colder.
02:39And some species of giant sloths were living their best lives back when Earth was transitioning between cold and warmer periods.
02:48Their big bodies usually protected these animals from temperature changes.
02:52But drastic fluctuations probably made them hide in order to survive.
02:57Besides, just like modern sloths, giant sloths probably had a hard time maintaining their body temperature.
03:02And going underground for a while is a great way to protect your babies and save some energy.
03:09After all, the bigger you are, the more energy you spend.
03:14Some scientists believe these tunnels might have had rotating tenants.
03:19One sloth moves in, digs a bit, leaves some scratch marks, moves out.
03:23And sometime later, another one crashes there and adds its own decor touches.
03:27A hostel for ancient megafauna.
03:31All evidence points to a more timeshare situation than a prehistoric private property.
03:36After all, these creatures weren't exactly social butterflies.
03:40Most sloths and armadillos usually keep things strictly one-on-one.
03:44Just mom and baby.
03:45Yeah, dad sloths stuck around only for the good part.
03:51But you probably noticed one tiny detail here.
03:54These guys are not around anymore.
03:56They went extinct 11,000 years ago, right around the time humans started to get popular on Earth.
04:03When another ice age was ending.
04:06In fact, 80% of all mammals that were heavier than 2 pounds disappeared then.
04:11The first culprit is, of course, the usual suspect.
04:16Humans.
04:17Around 10,000 years ago, a wave of skilled hunters using distinctive tools like spear points spread across North America.
04:24The theory goes that American animals weren't used to this new predator, making them easy targets.
04:30Once the big plant eaters started vanishing, the big meat eaters followed.
04:35They didn't have enough food to survive.
04:37And it wasn't just excessive hunting that caused some damage.
04:41Early humans may have also harmed ecosystems in ways that made survival harder for large species.
04:47But wait, there's another suspect.
04:51Climate change.
04:52Around the same time, Earth's climate got a major shakeup.
04:56The climate was going from super cold to only slightly cold.
05:00Just so you know, Earth's temperature was about 46 degrees Fahrenheit, making seasons change and plants perish.
05:06And if plants are no longer around, big animals that eat them, like our friends, giant sloths, have less nutrition, making them even more vulnerable.
05:18Many scientists now think that both human impact and climate change played a role, just in different ways, depending on the region.
05:25In some places, humans may have tipped the balance.
05:28In others, nature itself did the job.
05:31Either way, it was a perfect storm that ended the age of giants.
05:34Turns out, being huge isn't always a plus.
05:40Giant animals tend to live longer and reproduce slowly, which makes them sitting ducks when disaster strikes.
05:46Besides, it takes a long time for these guys to bounce back.
05:51After dinos disappeared, for example, it took 15 million years for new mega mammals to stomp onto the scene.
05:58The last major extinction happened around 12,000 years ago.
06:02Not really enough time for brand new massive beasts to evolve.
06:07Today's biggest ground animals, like elephants, rhinos, and bison, aren't newcomers.
06:12They're actually the tough survivors of that ancient chaos.
06:16And, well, at least we still live alongside the blue whale, the biggest mammal ever.
06:21But why did some prehistoric animals get so gigantic in the first place?
06:28Well, being big has some perks, in spite of all the extinction thing.
06:33It keeps you safe from predators and helps you win the fight for food.
06:37If plant eaters bulked up, predators had to level up, too, just to keep up.
06:42And the environment plays a big role.
06:45You see, cold-blooded animals might grow bigger in hot places to stay cool,
06:49and warm-blooded giants thrive in cold places to stay warm.
06:53One paleontologist even suggested that some dino-sized vegetarians needed those massive bellies
06:59just to digest the tough, woody plants of their time, as if they had a salad bar inside their bodies.
07:06There's also the thing with oxygen.
07:11Back then, Earth's atmosphere had way more of it.
07:14That's a big deal, especially for creatures with less efficient breathing systems, like insects.
07:20Higher oxygen levels meant their bodies could handle more growth and more activity,
07:24kind of like nature's turbo mode.
07:27We also have to discuss bones, specifically the lighter ones.
07:31Many huge dinosaurs had air sacs built into their skeletons,
07:35just like modern birds.
07:37This made their bones strong, but not ridiculously heavy,
07:40meaning they could grow super tall or long without collapsing under their own weight.
07:46Those air sacs also helped with cooling down,
07:49which is super important when you're basically a walking oven of muscle.
07:53On the flip side, modern mammals are limited by gravity and biology.
07:58Bigger bodies need stronger hearts, lungs, and blood pressure just to keep functioning,
08:03and land doesn't offer much support.
08:07This also explains why whales got even bigger than dinosaurs.
08:12The ocean acts like a natural support system,
08:14which pretty much means that buoyancy helps hold up all that mass,
08:18so blue whales can hit a jaw-dropping 330,000 pounds without turning into pancakes.
08:25On land, that kind of size would be a serious health hazard.
08:29So, ancient giants weren't just freaks of nature.
08:33They were the result of the right mix of air, food, physics, and environment working together in just the right way.
08:41The whole extinction event surely is a bummer for giant sloths that worked as gigantic engineers of nature,
08:47but it surely prevented us from sharing the planet with insects the size of crows,
08:51like some types of griffin flies or millipedes reaching almost 6 feet in length.
08:57Yikes!
08:58Now, if you'll excuse me, I'm going to appreciate modern sloths,
09:02slow, small, and thankfully incapable of terraforming the landscape and giving me insect-related panic attacks.
09:09That's it for today.
09:13So, hey, if you pacified your curiosity, then give the video a like and share it with your friends.
09:18Or, if you want more, just click on these videos and stay on the bright side!
