World’s largest spiderweb found in ‘Sulfur Cave’ housing 111,000 arachnids

World’s Largest Spiderweb Found in Dark Albanian-Greek Cave (Image: Urak et al. 2025, Subterranean Biology (CC BY 4.0))

Deep beneath the Albanian-Greek border, a hidden world has stunned scientists. Inside a pitch-black cave, researchers have uncovered what could be the world’s biggest spiderweb, stretching across the cavern walls like a living tapestry.

How did scientists uncover this vast web?
The discovery was made inside Sulfur Cave, a dark and narrow passage where spiders have spun an enormous network. Researchers described the web, spread over 1,140 square feet, as a patchwork of thousands of funnel-shaped homes. The study, published on 17 October in Subterranean Biology, called it an “extraordinary” colony.

What species live inside the web city?
According to lead author István Urák from the Sapientia Hungarian University of Transylvania in Romania, the colony includes two spider species: Tegenaria domestica, known as the barn funnel weaver, and Prinerigone vagans. Urák and his team estimated nearly 69,000 of the former and over 42,000 of the latter. DNA analysis confirmed both as dominant species.

A barn funnel weaver, also known as the domestic house spider (Tegenaria domestica), spotted inside Sulfur Cave. (Image: Urak et al. 2025, Subterranean Biology (CC BY 4.0))

Why is this discovery so surprising?
The two species were not known to share webs. Typically, Tegenaria domestica would prey on P. vagans, but the constant darkness may have altered their behaviour. The absence of light likely affects their vision, forcing cooperation instead of conflict. Urák described the moment of discovery as one of “admiration, respect, and gratitude.”

What sustains this spider colony?
The spiders feed on non-biting midges that thrive on microbial films formed by sulfur-oxidising bacteria. A sulphur-rich stream flows through the cave, filling it with hydrogen sulphide and sustaining both microbes and midges. This food chain, scientists say, allows the spiders to survive in a toxic environment.

The spiders in Sulfur Cave feed on non-biting midges that swarm in large numbers near the cave’s entrance. (Image: Urak et al. 2025, Subterranean Biology (CC BY 4.0))

Are the cave spiders genetically different?
Genetic analysis revealed that the spiders living inside the cave have diverged from those outside. Their microbiomes are also less diverse due to their sulphur-based diet. Urák noted that such adaptations show how species evolve in extreme environments.

What’s next for the researchers?
The team is now preparing another study to learn more about the colony’s ecology. Urák said preserving the site is vital, despite the cave’s challenging position between two countries. “Often, we think we know a species completely,” he said, “yet unexpected discoveries still await in the dark.”