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Beneath the surface of some of the world’s most Instagram-worthy destinations—centuries-old cities, lush tropical rainforests, and tiny European towns, just to name a few—lay an eerie antithesis to all the loveliness above. Gritty tunnels brimming with history and human bones, spine-chilling caverns where sadistic rituals once took place, ancient subterranean cities: There’s plenty more to explore when you look, quite literally, below the surface. If you’re hoping to stoke the adrenaline (and don’t mind getting a little dirty in the process) on your next trip, check out some of the creepiest underground places in the world. Just be sure to pack a flashlight.

Twist Through an Underground City in Turkey

In 1963, a resident of Derinkuyu decided that a remodel of his home was in order. When a wall collapsed during the renovation process, he found behind it the entrance to an 18-story underground city estimated to date back almost 3,000 years. In fact, numerous underground cities snake across Turkey, with Derinkuyu being the largest and most popular tourist destination. It’s known to have been used as refuge throughout Turkey’s tumultuous past, helping protect its citizens from various invasions and Roman persecution during the early days of Christianity.

Visitors brave enough to descend into this amazing underground city are only allowed to visit the first eight floors, but that’s plenty enough to get a feel for what life underground would have been like. The one-way only tour leads through hand-carved twisting passages, now illuminated with spotty electricity, where remains of the oil lamps that once lit the darkness can still be seen. Guests wind through kitchens, bedrooms, bathrooms, food storage rooms, oil and wine presses, wells, churches, schools, animal stables, and, yes, tombs that would have served up to 20,000 people for months—or even years.

Marvel at Glowing Fly Larvae in New Zealand

On the surface, it’s hard to imagine how a 30 million-year-old limestone cave that’s home to countless shiny maggots that shoot silky threads oozing with biophosphorecent mucus made the North Island of New Zealand famous. That is, until the Arachnocampa luminosa, or bioluminescent glow worms, begin to hunt—and the spectacular show begins at the Waitomo Glowworm Caves. When they are hungry, these larvae shine brightly, transforming the ceiling of the cave into a van Gogh-worthy version of a starry night sky.

Experience this mysterious underground world by caving, climbing, or boating in this underground cavern that could easily be mistaken for a high-tech movie set. Boat is, by far, the most popular way to explore the glowworm caves. The adventure is suitable for the whole family—that is, if everyone can get past the “glowing mucus” part and just focus on the visual splendor.

Learn About the Heavy History of the Vietnam War in Vietnam

One of Vietnam’s most notable sites, the Củ Chi tunnels offer an up-close look into the heavy history of the Vietnam War. Falco Ermert

Claustrophobes, heads-up: You’ll certainly be navigating some tight spots during a visit to one of Vietnam’s most notable sites, the Củ Chi tunnels. But if you’re up for the challenge, it’s an unforgettable experience. Most notable for their pivotal role in helping the Vietnamese win the Vietnam War, the tunnels are one of the country’s most popular and notable tourist spots, especially for history buffs.

Over the course of the war, the Viet Cong dug tens of thousands of miles of tunnels, including the hundred or so miles of the Cu Chi district northwest of Ho Chi Minh City (Saigon.) Soldiers, women, and children all used the tunnels as a safe haven, although they had to contend with vermin and parasitic infestations. In addition to being well hidden with tiny entrances, some no more than a foot or so wide, the tunnels were often booby-trapped with explosives or punji stake pits (often covered in human feces; if the spike didn’t kill, you sepsis often would.)

Today, the tunnels have been made safer for tourism, and several have been widened’ for Western visitors (though that doesn’t feel obvious while you’re crawling along on hands and knees, squeezing your torso through the tighter spots). The Cu Chi complex also offers visitors a sample of the tasteless, starchy cassava root that was the main staple of an underground diet. In addition, to get the full feel and sound of war, you can fire a war-era AK-47 or M-16 for about $1 U.S. per bullet.

Wonder at Piles of Human Bones in an Italian Crypt

Despite its tongue-twisting name, the Santa Maria della Concezione dei Cappuccini church is fairly unremarkable upon first glance, with its unadorned brown facade. Once below the chapel, however, the shock factor ramps up as you descend into macabre crypts where the bones of more than 4,000 friars adorn the walls and ceiling.

While some of the skeletons are intact and dressed in rough, brown Franciscan habits, the bones have been arranged into elaborate ornamental formations that would rival the decor of any palace in Italy—well, if they weren’t made out of human skeletons, that is. It’s said that they are arranged with religious significance, and the monks would come to pray and reflect. Even so, it’s almost impossible to walk through the crypt, which is divided into six pocket-sized chapels, without feeling a chill down your spine. Photography isn’t permitted, but to be honest, the views are sure to leave a lasting impression.

Watch a Massive Bat Exodus in Malaysia

At Gunung Mulu National Park, you can watch bats fly en mass from their subterranean homes. Alan Cressler

If one bat is spooky on its own, imagine the creep factor of millions of bats in chittering, fluttering clouds? Welcome to the Mulu Cave system, which is famous for its population of Wrinkle-lipped bats. It’s a popular activity at dusk to watch the spectacular mass exodus of these winged creatures heading out to hunt (bonus: Because there are so many bats in the area, mosquitoes are pleasantly scarce in the surrounding jungle.). The system is also famous for a few notable marks: Sarawak Chamber is believed to be the world’s largest known open chamber (2,300 feet long, 1,299 feet wide; and at least 230 feet high). It’s so big that it could accommodate about 40 Boeing 747s without touching. Nearby, Deer Cave is one of the largest single-cave passages in the world.

Bats may be the most famous animals in these parts, but other notable residents are thousands of swiftlets, whose nests are coveted for the Asian specialty bird’s nest soup. In addition, harmless but potentially startling cave racer snakes commonly make an appearance, and poisonous cave centipedes make gloves, which are handy for gripping the slippery, guano-splattered handrails, an even more vital necessity. But perhaps the most off-putting critters down here are the masses of cave roaches swarming in disturbing numbers over the bat guano, which appear the make the floor shift in the light of a dim headlamp.

The caves are located in the remote Gunung Mulu National Park, on the Malaysian side of the island of Borneo. It’s so remote, in fact, that it is only accessible by airplane, a 12-hour boat ride or a two-day hike along the infamous headhunters trail that actual headhunters used on the warpath. It’s even possible to book a tour with a local guide who is a recent descendant of those same fierce headhunters to see what life is like in a modern longhouse (the traditional communal homes of natives). And if you’re really lucky, you may also have a chance to try blowing “poison” darts.

Explore a Former Abandoned Mine in Wales

Under the spectacular mountains of Snowdonia National Park are the lonesome remnants of the Cwmorthin Quarry and Wales mining industry. Opened in 1810, this rugged shale mountain was difficult to mine, and it claimed the lives of numerous workers, earning it the ominous moniker “The Slaughterhouse” as a result. Today, a group of forward-thinking entrepreneurs has turned this historic spot into an extreme adventure playground for the daring.

Brave participants begin the adventure staring into the inky darkness of a nine-story drop into the earth. The full adventure takes the brave 1,312 feet underground—approximately about 100 feet deeper than the Empire State Building is tall, to put that into perspective. Along three miles through this massive slate mine, visitors will tackle nine zip lines (including the world’s deepest), a 72-foot rappel, free fall, and numerous via ferrata-style climbs. The mine has been left mostly as it’s been since its inception, with no concrete floors or electric lights. Caverns and shafts are still littered with historic remnants like railway cars, tallow candles, and a leather miner’s cap.

See Actual Mayan Sacrifices in Belize

The Crystal Maiden, as this skeleton is known for the way its crystallized bones sparkle in the light, is just one of the Mayan sacrifices visitors see along the way in the ATM cave. Antti T. Nissinen

The Mayan civilization is well known for having brought the world a highly advanced calendar, spectacular art, and impressive architecture. But the culture also had a darker side that involved brutal, ritual sacrifice of human lives, including children.

Adventure-minded travelers can get an up-close look at that living history with a tour through a cave known as Actun Tunichil Muknal, also known as ATM. Deep in the Tapir Mountain Nature Reserve, located in the foothills of the Maya Mountains in western Belize, the cave’s remote location makes just getting there an adventure, with an hour-long hike through dense jungle while fording shallow rivers. At the entrance, you’ll plunge into a river into the cave before wading through water—and several tight spots—deep into the caverns strewn with human bones and pottery shards. The Mayans believed these eerie caves were the entrance to the underworld where gods lived. In fact, the local nickname of this particular cave is “a place of fear”, even 1,000 years after its last unlucky inhabitant lost his or her life in the name of appeasing the gods.

Indeed, the cave is the final resting place for the remains of 14 people believed to have been sacrificed to bring bounty upon the land. Scientists estimate they range in age from one year to adults, with several children among the number. The most famous is the “Crystal Maiden”, a young woman of about 18, limbs askew, whose bones have begun to crystallize into the rock. It’s a spectacular, humbling sight, indeed—and one that might haunt you long after you emerge from ATM’s subterranean spaces and into the bright Belize sunshine.

Written by Lisa Collard for RootsRated and legally licensed through the Matcha publisher network. Please direct all licensing questions to legal@getmatcha.com.