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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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Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
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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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Cs California, Wikimedia Commons // CC BY-SA 3.0
How Experts Say We Should Stop a 'Zombie' Infection: Kill It With Fire
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Cs California, Wikimedia Commons // CC BY-SA 3.0

Scientists are known for being pretty cautious people. But sometimes, even the most careful of us need to burn some things to the ground. Immunologists have proposed a plan to burn large swaths of parkland in an attempt to wipe out disease, as The New York Times reports. They described the problem in the journal Microbiology and Molecular Biology Reviews.

Chronic wasting disease (CWD) is a gruesome infection that’s been destroying deer and elk herds across North America. Like bovine spongiform encephalopathy (BSE, better known as mad cow disease) and Creutzfeldt-Jakob disease, CWD is caused by damaged, contagious little proteins called prions. Although it's been half a century since CWD was first discovered, scientists are still scratching their heads about how it works, how it spreads, and if, like BSE, it could someday infect humans.

Paper co-author Mark Zabel, of the Prion Research Center at Colorado State University, says animals with CWD fade away slowly at first, losing weight and starting to act kind of spacey. But "they’re not hard to pick out at the end stage," he told The New York Times. "They have a vacant stare, they have a stumbling gait, their heads are drooping, their ears are down, you can see thick saliva dripping from their mouths. It’s like a true zombie disease."

CWD has already been spotted in 24 U.S. states. Some herds are already 50 percent infected, and that number is only growing.

Prion illnesses often travel from one infected individual to another, but CWD’s expansion was so rapid that scientists began to suspect it had more than one way of finding new animals to attack.

Sure enough, it did. As it turns out, the CWD prion doesn’t go down with its host-animal ship. Infected animals shed the prion in their urine, feces, and drool. Long after the sick deer has died, others can still contract CWD from the leaves they eat and the grass in which they stand.

As if that’s not bad enough, CWD has another trick up its sleeve: spontaneous generation. That is, it doesn’t take much damage to twist a healthy prion into a zombifying pathogen. The illness just pops up.

There are some treatments, including immersing infected tissue in an ozone bath. But that won't help when the problem is literally smeared across the landscape. "You cannot treat half of the continental United States with ozone," Zabel said.

And so, to combat this many-pronged assault on our wildlife, Zabel and his colleagues are getting aggressive. They recommend a controlled burn of infected areas of national parks in Colorado and Arkansas—a pilot study to determine if fire will be enough.

"If you eliminate the plants that have prions on the surface, that would be a huge step forward," he said. "I really don’t think it’s that crazy."

[h/t The New York Times]