According to a study published last month in the journal Subterranean Biology, the Cave of Sulphur, a pitch-black cave in Southern Europe, is home to a remarkable colony – a spider megalopolis built by two spider species living together in a unique ecosystem. The enormous web stretches 106 square meters along the wall of a narrow passage near the cave entrance and consists of thousands of individual, funnel-shaped tissues.

“The natural world still holds countless surprises for us,” István Urák, an associate professor of biology at Sapientia Hungarian University of Transylvania in Romania, told Live Science. “If I were to attempt to put into words all the emotions that surged through me [when I saw the web], I would highlight admiration, respect, and gratitude. You have to experience it to truly know what it feels like.”

The world’s largest spider web is inhabited by two species of arachnids, Tegenaria domestica, known as the barn funnel weaver or domestic house spider, and Prinerigone vagans. Interestingly, scientists would expect the barn funnel weaver to prey on P. vagans, but the two species appear to be living in harmony in the cave. Although we don’t have a clear reason for why that is, researchers theorize that the lack of light in the cave may be impairing the spiders’ vision.

Both spider species feed on non-biting midges, which in turn feast on white microbial biofilms from sulfur-oxidizing bacteria in the cave. Gut Analysis showed that the sulfur-rich diet causes the cave dwellers’ microbiomes to be considerably less diverse than the microbiomes of spiders from the same two species outside the cave.

A priceless find for scientists and naturalists, the world’s largest spider web is a real-life nightmare for arachnophobes, and it’s likely to inspire a spider-themed horror film. Its location, an underground maze of huge chambers and winding tunnels, would undoubtedly crank the viewers’ adrenaline to 100.

Featured image credit: I. Urak et al. / Subterranean Biology / 2025 (CC BY 4.0)