The cavern along the border of Greece and Albania is home to a terrifyingly high number of two species of arachnids that live together peacefully in complete darkness
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Sarah Kuta - Daily Correspondent
November 6, 2025 4:30 p.m.
The giant structure—made up of thousands of individual funnel-shaped webs—spans over 1,000 square feet. Marek Audy
It may sound like something out of a nightmare, but scientists say they weren’t dreaming when they discovered a massive spiderweb that’s home to more than 110,000 arachnids inside a cave in …
The cavern along the border of Greece and Albania is home to a terrifyingly high number of two species of arachnids that live together peacefully in complete darkness
Sarah Kuta - Daily Correspondent
November 6, 2025 4:30 p.m.
The giant structure—made up of thousands of individual funnel-shaped webs—spans over 1,000 square feet. Marek Audy
It may sound like something out of a nightmare, but scientists say they weren’t dreaming when they discovered a massive spiderweb that’s home to more than 110,000 arachnids inside a cave in southeastern Europe. It’s one of the largest spider colonies ever documented, possibly the biggest in the world, researchers report in a paper published October 17 in the journal Subterranean Biology.
Researchers found the “extraordinary,” enormous spider community in Sulfur Cave, a cavern along the border of Albania and Greece. The spiderweb was initially discovered in 2022 by speleologists—cave scientists—with the Czech Speleological Society, prompting biologists to plan their own expeditions for a closer look.
When lead author István Urák, a biologist at Sapientia Hungarian University of Transylvania, first laid eyes on the colossal web, he felt a surge of emotions ranging from gratitude to respect. “You have to experience it to truly know what it feels like,” he tells Live Science’s Sascha Pare.
Key takeaway: The undiscovered underworld of caves
Scientists and explorers alike still have much to find inside Earth’s abundant caves. According to speleologist Patricia Kambesis, now director of the American Cave Conservation Association, caves “have not generally attracted the attention of mainstream scientists” despite their potential value for systematic exploration and scientific inquiry.
The huge structure—made up of thousands of individual funnel-shaped webs—spans over 1,000 square feet. It starts 164 feet from the cave’s entrance and extends through a narrow, low-ceilinged section. The arachnids live in total darkness, as their neighborhood never receives sunlight.
Two species of spiders inhabit this dark, murky world: Tegenaria domestica, known as the barn funnel weaver or domestic house spider, and Prinerigone vagans.
Scientists estimate some 69,000 T. domestica and more than 42,000 P. vagans are living in the cave. They also found an “unusually dense” swarm of small midge flies called Tanytarsus albisutus in the cavern, which likely serve as the main food source for the arachnids, they write in the paper.
The team was surprised to find these spider species living in the cave, as they’re typically found on the surface. Beyond that, it’s one of the first recorded instances of the two species living together in apparent harmony. The scientists believe the *T. domestica *spiders are the primary architects and builders of the giant web, while P. vagans is simply living in it.
The researchers would have expected the much larger T. domestica spiders to eat the smaller P. vagans, but perhaps the cave’s darkness is responsible for their congeniality, they speculate. This theory seems especially plausible given that *P. vagans *spiders tend to remain motionless until their own prey is within striking distance. Perhaps, they speculate, the T. domestica simply cannot see them.
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Scientists believe the *T. domestica *spiders are the primary architects and builders of the large web. Urák et al. / Subterranean Biology, 2025
More broadly, the investigation revealed the spiders living in Sulfur Cave are unique. They’re genetically distinct from other members of their species, likely because they’ve adapted to life in the dank, dim cave. In addition, the cave spiders’ gut microbiomes are less diverse than their peers living outside the cavern, which is likely a result of their diet.
Both findings were “unexpected discoveries,” Urák tells Live Science. “Some species exhibit remarkable genetic plasticity, which typically becomes apparent only under extreme conditions,” he says.
Sulfur Cave’s entrance is located in Greece, while its deepest sections are in Albania. It joins up with two other caverns—Atmos Cave and Turtle Cave—to create a subterranean maze of rooms and passages in a limestone outcrop. Water from springs and a sulfur-laden stream flows through the cave complex and out into the Sarandaporo River, with water temperatures inside the cave hovering around 78.8 degrees Fahrenheit year-round.
The cave walls are covered in slimy biofilms, made up of microorganisms that feed on the abundant hydrogen sulfide in the cave. These biofilms form the basis of the cavern’s food web, allowing numerous critters to thrive—the two species of spiders and the midges, but also centipedes, pseudoscorpions, mites and beetles.
Researchers are continuing to study the cave’s creepy-crawly residents. But in the meantime, Urák tells Live Science, they’re reveling in the fact that “the natural world still holds countless surprises for us.“
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