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12 Amazing Underground Destinations to Visit

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Instead of the same old beach vacation this summer, why not beat the heat by heading underground? From amusement parks in salt mines to subterranean gardens, there are a variety of fascinating underground travel destinations where you can relax, see the sights, and cool off.


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Salina Turda (Turda Salt Mine) was a salt mine in Transylvania that operated for hundreds of years, up until 1932, when it was repurposed to serve as cheese storage and as a bomb shelter. In 1992, parts of the mine were converted into something more fun: an amusement park.

At 400 feet below ground, the park features an amphitheater, mini-golf, basketball courts, a bowling alley, a Ferris wheel, a carousel, and a spa. The latter takes advantage of the chambers' supposedly health-giving temperatures—a constant 54 degrees with 80% humidity—and relative freedom from allergens and bacteria. Tourists can also rent rowboats and paddle around on an underground lake.



Leo Lambert bought land above the Lookout Mountain Caverns in Tennessee in 1928, hoping to open a new entrance to the caves and make some money from tourists. Instead, he found an entirely new cave that contained its own underground waterfall. Lambert named the cave and its waterfall after his wife, Ruby. Ruby Falls, located in Ruby Falls Cave, is now one of Tennessee's best-known tourist attractions.


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Cité souterraine de Naours, or the Underground City of Naours, was originally a limestone quarry. Many centuries ago, local residents in this part of northern France discovered that the underground chambers were a great place to store supplies, hide from raiders, and find shelter from the elements. By the 17th century, the chambers had been turned into a city of 3000 people, complete with homes, chapels, businesses, meeting rooms, and even livestock facilities. Cité souterraine de Naours was abandoned as the area grew more peaceful, but was rediscovered in 1887.


Gordito1869 via Wikimedia Commons // CC BY 3.0

Residents of the Andalucían town of Guadix began carving their homes out of limestone hundreds of years ago. They're usually recognizable by the chimneys that arise from the ground, with no houses visible underneath (some homes also have visible exteriors, but extend much further underground). Many of these homes were first built for protection from invaders, yet they are so comfortable and economical that they are still used today. While the private homes of Guadix aren't open to the public, the proud owners might grant you a peek. If all else fails, you can visit the Cave Dwellings Interpretation Centre in Guadix and learn about the history of the homes.


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The town of Coober Pedy, South Australia grew up around the opal mining industry. But the desert conditions are so extreme that Mad Max Beyond Thunderdome was filmed there. Daytime temperatures regularly reach into the triple digits, and can reach 113°F in the shade during summer. To deal with the extreme heat, the residents tend to build their homes underground. Abandoned chambers that have been mined of their opals are also used for underground community buildings.

The Umoona Opal Mine & Museum is the town's largest underground attraction, and a good place to learn about the history of Coober Pedy and its opal mines. The town's Serbian Orthodox Church is also quite notable, with its sanctuary carved by volunteers under a sandstone hill, along with a fellowship hall, school, and parish house—all underground. The church is open to visitors.



The Puerto-Princesa Subterranean River National Park, a UNESCO World Heritage Site, surrounds one of the longest underground rivers in the world. The 5-mile underground section of the river empties into the ocean, after flowing through a limestone cave full of spectacular formations and populated by bats, monkeys, sea snakes, and other wildlife. Tour company PPUR offers excursions to the park starting from Puerto Princesa City, about 45 minutes away.


Carol M. Highsmith via Wikimedia Commons // Public Domain

Sicilian Baldassare Forestiere immigrated to California around the turn of the 20th century, but soon found the heat of Fresno oppressive. Inspired by the catacombs near his home in Sicily, he began digging himself underground caverns in which to stay cool, and kept digging for the next 40 years. Along the way, Forestiere developed methods to deliver enough sunshine so that he could grow fruit trees and grapevines in his underground home. The result of his digging is now Forestiere Underground Gardens, where some of his original trees are still thriving 100 years later.



After Edinburgh's South Bridge was completed in 1788, the space under its 19 massive stone arches was used for storage and small businesses such as taverns and cobblers. But as conditions beneath the bridge deteriorated (for one thing, it had never been properly waterproofed), businesses moved out and squatters, criminals, and fugitives moved in. The maze of rooms and corridors now known as the Edinburgh Vaults were rediscovered during an excavation in 1985, when evidence of past residents sparked interest in South Bridge history. The vaults are now a tourist draw, and it doesn't hurt a bit that the mysterious underground corridors are said to be haunted by the ghosts of those who used them.



In the 6th century, Byzantine Emperor Justinianus I constructed a huge underground reservoir near the southwest corner of the Hagia Sophia in Istanbul. It was built beneath the Stoa Basilica, why is why it's known as the Basilica Cistern, or Yerebatan Sarnici in Turkish. The brick-lined chamber is supported by 336 stone columns, and can hold up to an estimated 100,000 tons of water. The cistern has been abandoned and then restored several times over the centuries, most recently in 1985. The most striking features of the cistern's architecture are the two giant Medusa heads that support two of the columns. One is placed sideways, while the other is upside down. They may have been taken from a Roman building, and placed in their unusual manner to show disrespect for pagan figures. The Basilica Cistern is now a museum, open to the public.



The volcanic activity that led to the birth of Japan's Mt. Fuji also produced caves in the ground underneath the mountain. One of these is Narusawa Hyoketsu Ice Cave in Yamanashi Prefecture. The cave is near the east entrance of the Aokigahara Jukai forest, which is also known as the "suicide forest." Narusawa Hyoketsu is so cold that water dripping from the ceiling forms pillars of icicles year-round. The average temperature inside the cave is barely above freezing, which made the cave a perfect place to store ice before mechanical refrigeration. Today, it's open for visitors.



A former underground limestone quarry in Louisville, Mega Cavern spans about 4 million square feet. Much of that space is dedicated to a commercial storage business, but you might be more interested in the underground amusement park. The park includes the a 320,000-square foot bike park, a zip line, an aerial rope challenge course, and tours.


Daniel.zolopa via Wikimedia Commons // CC BY-SA 3.0 pl

The Wieliczka Salt Mine near Krakow produced salt from the 13th century up until 2007, but even before mining ceased it became a national historic monument and a tourist attraction. The natural resource has figured prominently in Poland's history over the centuries, and miners created an underground world that includes chapels and artworks carved into the walls, with more added by modern artists. Wieliczka Salt Mine now has hundreds of miles of underground corridors, shafts, and chambers to see. Visitors to the UNESCO World Heritage Site can choose between several tours of various lengths and themes, visit the museum, relax at the underground health spa, have an underground meal, or see concerts and other events in the mine's chambers.

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iStock // Ekaterina Minaeva
Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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Sponsor Content: BarkBox
8 Common Dog Behaviors, Decoded
May 25, 2017
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Dogs are a lot more complicated than we give them credit for. As a result, sometimes things get lost in translation. We’ve yet to invent a dog-to-English translator, but there are certain behaviors you can learn to read in order to better understand what your dog is trying to tell you. The more tuned-in you are to your dog’s emotions, the better you’ll be able to respond—whether that means giving her some space or welcoming a wet, slobbery kiss. 

1. What you’ll see: Your dog is standing with his legs and body relaxed and tail low. His ears are up, but not pointed forward. His mouth is slightly open, he’s panting lightly, and his tongue is loose. His eyes? Soft or maybe slightly squinty from getting his smile on.

What it means: “Hey there, friend!” Your pup is in a calm, relaxed state. He’s open to mingling, which means you can feel comfortable letting friends say hi.

2. What you’ll see: Your dog is standing with her body leaning forward. Her ears are erect and angled forward—or have at least perked up if they’re floppy—and her mouth is closed. Her tail might be sticking out horizontally or sticking straight up and wagging slightly.

What it means: “Hark! Who goes there?!” Something caught your pup’s attention and now she’s on high alert, trying to discern whether or not the person, animal, or situation is a threat. She’ll likely stay on guard until she feels safe or becomes distracted.

3. What you’ll see: Your dog is standing, leaning slightly forward. His body and legs are tense, and his hackles—those hairs along his back and neck—are raised. His tail is stiff and twitching, not swooping playfully. His mouth is open, teeth are exposed, and he may be snarling, snapping, or barking excessively.

What it means: “Don’t mess with me!” This dog is asserting his social dominance and letting others know that he might attack if they don’t defer accordingly. A dog in this stance could be either offensively aggressive or defensively aggressive. If you encounter a dog in this state, play it safe and back away slowly without making eye contact.

4. What you’ll see: As another dog approaches, your dog lies down on his back with his tail tucked in between his legs. His paws are tucked in too, his ears are flat, and he isn’t making direct eye contact with the other dog standing over him.

What it means: “I come in peace!” Your pooch is displaying signs of submission to a more dominant dog, conveying total surrender to avoid physical confrontation. Other, less obvious, signs of submission include ears that are flattened back against the head, an avoidance of eye contact, a tongue flick, and bared teeth. Yup—a dog might bare his teeth while still being submissive, but they’ll likely be clenched together, the lips opened horizontally rather than curled up to show the front canines. A submissive dog will also slink backward or inward rather than forward, which would indicate more aggressive behavior.

5. What you’ll see: Your dog is crouching with her back hunched, tail tucked, and the corner of her mouth pulled back with lips slightly curled. Her shoulders, or hackles, are raised and her ears are flattened. She’s avoiding eye contact.

What it means: “I’m scared, but will fight you if I have to.” This dog’s fight or flight instincts have been activated. It’s best to keep your distance from a dog in this emotional state because she could attack if she feels cornered.

6. What you’ll see: You’re staring at your dog, holding eye contact. Your dog looks away from you, tentatively looks back, then looks away again. After some time, he licks his chops and yawns.

What it means: “I don’t know what’s going on and it’s weirding me out.” Your dog doesn’t know what to make of the situation, but rather than nipping or barking, he’ll stick to behaviors he knows are OK, like yawning, licking his chops, or shaking as if he’s wet. You’ll want to intervene by removing whatever it is causing him discomfort—such as an overly grabby child—and giving him some space to relax.

7. What you’ll see: Your dog has her front paws bent and lowered onto the ground with her rear in the air. Her body is relaxed, loose, and wiggly, and her tail is up and wagging from side to side. She might also let out a high-pitched or impatient bark.

What it means: “What’s the hold up? Let’s play!” This classic stance, known to dog trainers and behaviorists as “the play bow,” is a sign she’s ready to let the good times roll. Get ready for a round of fetch or tug of war, or for a good long outing at the dog park.

8. What you’ll see: You’ve just gotten home from work and your dog rushes over. He can’t stop wiggling his backside, and he may even lower himself into a giant stretch, like he’s doing yoga.

What it means: “OhmygoshImsohappytoseeyou I love you so much you’re my best friend foreverandeverandever!!!!” This one’s easy: Your pup is overjoyed his BFF is back. That big stretch is something dogs don’t pull out for just anyone; they save that for the people they truly love. Show him you feel the same way with a good belly rub and a handful of his favorite treats.

The best way to say “I love you” in dog? A monthly subscription to BarkBox. Your favorite pup will get a package filled with treats, toys, and other good stuff (and in return, you’ll probably get lots of sloppy kisses). Visit BarkBox to learn more.