Inside the world's largest spiderweb hidden in a cave

A colossal secret has been unveiled in a cave — one that challenges our understanding of spider behaviour and cooperation. 

Scientists have discovered what may be the largest spiderweb ever documented, a sprawling “megacity” of silk, spanning over 106 square meters (1,140 square feet) and teeming with more than 111,000 arachnids. 

They found it in the dim, acrid depths of a cave straddling the rugged border between Albania and Greece. 

This isn’t the work of a single species but a bizarre alliance between two typically antagonistic spiders — thriving in an environment of total darkness, toxic fumes, and endless insect feasts.

The revelation, which surfaced in scientific circles earlier this month, has captivated explorers and biologists alike.

 A viral X post from user @chi1vrs brought the story to a wider audience, sharing footage of a caver tentatively reaching out to touch the web's spongy surface.

In the clip, the explorer — clad in a red helmet and yellow backpack — extends a gloved hand toward the glistening wall, illuminated only by the faint beam of a headlamp. 

What looks at first glance like a dripping mineral deposit is, in fact, a living tapestry of silk, pulsating with the hidden activity of thousands of spiders.

The web resides deep within Sulfur Cave, a labyrinthine network carved into limestone by the Sarantaporos River in Greece's Vromoner Canyon. 

The cave's entrance lies in Greece, but its passages snake across the border into Albania, making this find a true transboundary marvel. 

About 160 feet (50 meters) from the entrance, where sunlight vanishes and hydrogen sulfide fumes thicken the air, the massive sheet unfurls across a narrow passage. 

The environment is unforgiving: pitch-black, humid, and reeking of rotten eggs from the sulfur deposits that give the cave its name.

The initial sighting dates back to 2022, when a team of spelunkers from the Czech Speleological Society stumbled upon the anomaly during a routine expedition. 

“It was unlike anything we'd seen,” recalled one member in expedition logs. 

But it wasn't until 2024 that a dedicated scientific team, led by Hungarian biologist István Urák, ventured in to collect samples. 

Urák's group confirmed the web's scale and conducted genetic and microbiome analyses, publishing their findings in a peer-reviewed paper that has since made headlines.

Measuring roughly the size of half a tennis court, the web isn't a haphazard tangle but a structured funnel-sheet design, with silk threads radiating from central hubs like spokes on a wheel. 

Its surface feels spongy and moist to the touch — hence the caver's cautious probe in the video — thanks to the cave's high humidity and constant drip of mineral-rich water. 

Embedded within this silken expanse are over 2.4 million midges, the spiders' primary food source, providing a perpetual buffet that sustains the colony's immense population.

What makes this discovery truly extraordinary isn't just the size, but the inhabitants.

Genetic sampling revealed two distinct spider species coexisting in harmonious anarchy: the larger Tegenaria domestica (commonly known as the barn funnel weaver or house spider) and the smaller Prinerigone vagans (a moisture-loving linyphiid spider). 

Of the 111,000 residents, about 69,000 are T. domestica, while over 42,000 are P. vagans.

On the surface, this partnership defies logic. T. domestica is a notorious predator, often preying on smaller spiders like P. vagans in the wild. 

Yet here, in the cave's inky void, they've forged a colonial society—the first documented case of such behavior for either species. 

Researchers hypothesise that the total absence of light plays a pivotal role. 

“But in the cave, because it’s dark in there, our hypothesis was that they do not see each other,” explained Blerina Vrenozi, a biologist and ecologist at the University of Tirana in Albania, who contributed to the study.

Further analysis uncovered profound adaptations. 

The cave spiders exhibit genetic plasticity, with DNA differing from their surface-dwelling relatives, suggesting rapid evolution to the extreme conditions. 

Their microbiomes are strikingly less diverse, likely a defense against the cave's toxic gases. "Often, we think we know a species completely, that we understand everything about it, yet unexpected discoveries can still occur," Urák reflected. "Some species exhibit remarkable genetic plasticity, which typically becomes apparent only under extreme conditions."

This arachnid metropolis isn't just a curiosity—it's a window into evolutionary resilience. 

Caves like Sulfur Cave act as natural laboratories, isolating species and accelerating adaptations. 

The spiders' success here underscores how environmental pressures can flip traditional predator-prey dynamics on their heads. 

In a world grappling with habitat loss and climate change, such findings remind us of nature's capacity for innovation.

For spelunkers and arachnologists, the site poses new questions: How stable is this colony? Could similar "megacities" lurk in other unexplored caves? Conservation efforts are already underway, with Greek and Albanian authorities coordinating to protect the area from tourism or mining threats.