Scientists have identified what may be the world’s biggest spiderweb ever recorded — a massive structure housing more than 111,000 spiders inside a pitch-dark cave known as Sulfur Cave, located on the Greek-Albanian border. The cave’s entrance is in Greece, while its deep sections are in Albania.
The finding, published in the journal Subterranean Biology, describes the web as an “extraordinary” network that spans 1,140 square feet (106 square meters) along the wall of a narrow, low-ceilinged passage. The structure is made of thousands of fused, funnel-shaped webs woven into a single colony-wide sheet that never receives natural light.
Lead author István Urák, a biology professor at Sapientia Hungarian University of Transylvania in Romania, said the discovery marks the first documented case of colonial living in two widespread spider species. He called the site “a unique case of two species cohabiting within the same web structure in these huge numbers.”
Web first spotted by cavers in 2022
The massive web sits inside Sulfur Cave, a chamber formed when sulfuric acid dissolved the surrounding rock. Members of the Czech Speleological Society first noticed the web in 2022 during an expedition through the Vromoner Canyon. A research team returned in 2024 to collect specimens, which Urák later analyzed before carrying out his own fieldwork inside the cave.
Genetic testing confirmed that the colony consists of Tegenaria domestica, also known as the barn funnel weaver or domestic house spider, and Prinerigone vagans. Researchers estimated about 69,000 individuals of T. domestica and more than 42,000 of P. vagans living inside the shared structure.
Unusual cohabitation inside a lightless cave
Both species are common near human buildings, but they have never been documented sharing a web in the wild. Under normal circumstances, T. domestica would likely prey on P. vagans. The study suggests that total darkness may impair the spiders’ vision, reducing predatory behavior and allowing both species to coexist.
Instead of turning on each other, the spiders feed on non-biting midges that fill the chamber. The midges survive by eating white microbial biofilms — slimy protective layers produced by sulfur-oxidizing bacteria.
A sulfur-rich stream runs through the cave, releasing hydrogen sulfide that fuels the bacteria, the midges, and the spiders that feed on them. Researchers describe the site as a food chain powered entirely by sulfur, not sunlight.
Evidence of genetic and microbial adaptation
Gut content tests showed that the spiders’ sulfur-based diet has reshaped their microbiomes, making them far less diverse than those of the same species living outside the cave. DNA analysis also revealed genetic differences, suggesting the colony has already begun adapting to the extreme conditions inside Sulfur Cave.
Urák said the discovery shows that even familiar species can behave in unexpected ways when forced into harsh environments. He said some animals display “remarkable genetic plasticity,” but it becomes visible only when their surroundings impose pressure.
Scientists call for protection of the cave ecosystem
Urák stressed the importance of preserving the site, noting that its location across two national borders may complicate conservation plans. He added that the research team is already preparing a follow-up study to learn more about the sulfur-driven ecosystem hidden inside the cave.