Even in the darkness of the cave, it's hard to miss what is believed to be the world's largest spider web.
At about 1,140 square feet, it's the size of a small house and hangs in a low, narrow passageway in a cave that straddles the Albanian-Greece border.
But what scientists recently discovered in Sulfur Cave, a network of rooms and passageways carved out of limestone along the Sarantapoulos River, surprised them even more than the size of the nest.
In the spider metropolis of 111,000 people, two previously unknown species coexisted in harmony. The main reason for this was that one species tends to eat the other.
The team of scientists found that 69,000 Tegenaria domestica, known as barn funnel weavers, coexisted with around 42,000 Prineligone vagrants, which live in damp areas. Barn funnel weavers typically prey on smaller P. vagans.
“But our hypothesis was that they wouldn't see each other because it was dark inside the cave,” Blerina Brenoj, a biologist, zoologist and ecologist at Albania's University of Tirana, said in an interview. “So they don't attack.”
Dr. Vrenoj is one of the scientists who published his research on sulfur caves in a peer-reviewed paper. Last month's Subterranean Biology magazine.
“I've been involved in spider research for 18 years and I've never seen a community like this,” she said.
Dr Brenosi said it was “not that easy” to cross the river through chest-high water to reach the cave while wearing a wetsuit and boots and clinging to a rope. But she added that it was “pure adrenaline for biologists.”
She said that when the light shone on the cave, it appeared to sparkle. The wider web is actually a pastiche of thousands of funnel-shaped webs, which “really bright when the light hits them because the silk dances over them, so you could see this surface in many dots, like lights in a giant web,” she said.
The cave itself was hollowed out by sulfuric acid, formed by the oxidation of hydrogen sulfide in groundwater. The Czech Speleological Society discovered and reported this cave in 2022. The team of researchers, who published their findings last month, then visited the cave several times between 2023 and 2025.
The cave is kept at about 80 degrees all year round. The huge population of spiders is thought to be due to the abundance of food available. There are more than 2.4 million midges in the cave, and at any moment they can become entangled in intricate nests.
The environment is also unusually protected. The cave is difficult to reach and filled with foul-smelling hydrogen sulfide gas at concentrations too high for most animals to live there.
“You can only smell the sulfur-hydrogen and you can't breathe,” Dr. Brenosi said, recalling that most of the researchers were wearing masks. But as you go deeper into the cave, “you get used to the smell of rotten eggs,” she said.
Dr. Vrenosi and her colleagues don't know how long this spider metropolis has been around, but she said its remoteness means it could persist indefinitely.
“The web has many layers and is too heavy to stay attached to the wall, so parts of it may fall off,” she says. “But this is a cycle that repeats itself over and over again.”
Dr Vrenoj said: “Neither of these species are social, but solitary species, so cooperation between the two spider species will continue to be studied.”
Dinesh Rao, a spider biologist at the University of Veracruz in Mexico who was not involved in the study, said the estimated number of spiders is very high. He added that while the methodology appeared to be sound, the researchers may have overestimated the overall size of the webs because they included old, unused spider webs. (The study itself acknowledges this.)
The study also showed that spiders inside the cave were genetically distinct from their relatives outside, suggesting that spider metropolitan residents were adapting to their environment to build unique communities.
When Dr. Vrenosi reached out and touched the spider's web, he noticed that it was very soft and bounced back. According to her, it was “very spongy.”
