A Vast Spider Metropolis Hidden Beneath Europe

Deep within a sulfur-rich cave straddling the remote border between Albania and Greece, scientists have uncovered one of the most extraordinary arachnid communities ever documented. Hidden far from daylight, an immense, interconnected web system stretches across roughly 1,140 square feet and is inhabited by an estimated 111,000 spiders. What makes this discovery especially remarkable is not only its sheer scale, but also the unexpected coexistence of two spider species that would rarely tolerate one another under normal circumstances.

The colony is composed primarily of Tegenaria domestica, a species commonly known as the house spider, and Prinerigone vagans, a much smaller sheet-weaving spider. In surface environments, these species typically compete for space and prey. Yet inside the pitch-black cave, where sunlight never penetrates and conditions are chemically extreme, competition appears to have been replaced by a form of coexistence that borders on cooperation. Researchers believe that the absence of light and predators, combined with limited but stable food resources, has reshaped the spiders’ behavior, allowing them to build overlapping webs and tolerate close proximity.

Unlike most ecosystems on Earth, this underground environment is not driven by photosynthesis. Instead, it is sustained by sulfur-oxidizing bacteria that draw energy from chemical reactions involving sulfur compounds present in an underground stream. These bacteria form dense microbial mats, which in turn provide food for non-biting midges. The spiders occupy the top of this unusual food chain, feeding almost exclusively on these insects. Similar sulfur-based ecosystems have been documented in places such as Movile Cave in Romania, one of the world’s best-known chemosynthetic cave systems (National Geographic; Nature Ecology & Evolution).

Genetic and DNA analyses have revealed another layer of adaptation. The cave-dwelling spiders possess a gut microbiome that is distinctly different from that of their surface-dwelling relatives. Scientists suggest that this specialized microbiome helps the spiders digest prey that ultimately depends on sulfur bacteria, demonstrating how even familiar species can undergo subtle but important biological changes when exposed to extreme environments. Such findings align with broader research showing that microbiomes play a crucial role in helping animals adapt to harsh or unusual diets (Science, Proceedings of the National Academy of Sciences).

Beyond its biological novelty, the discovery challenges long-held assumptions about spider behavior. Spiders are typically considered solitary and territorial, yet this massive web network functions almost like a shared infrastructure, enabling thousands of individuals to coexist in close quarters. While the spiders are not truly social in the way ants or bees are, the structure of the colony suggests that environmental pressure can push even solitary species toward unexpected social-like arrangements (BBC Earth; Scientific American).

Researchers emphasize that this cave ecosystem deserves special protection. Its size, the rare interaction between species, and its reliance on sulfur-based energy make it virtually unique in Europe. Human disturbance, pollution, or changes in groundwater chemistry could easily disrupt the delicate balance that sustains the entire system. More broadly, the discovery highlights how little is still known about subterranean ecosystems and how many surprises remain hidden beneath our feet.

Ultimately, this underground spider metropolis serves as a powerful reminder that life is remarkably adaptable. Even creatures as familiar as house spiders can evolve new behaviors and biological traits when pushed into extreme and isolated environments, reshaping our understanding of where and how complex life can thrive.