While exploring a sulfur cave on the Albania-Greece border, scientists at the Czech Speleological Society discovered the largest spider web ever recorded. The vast “megacity,” spanning more than 1,040 square feet —nearly half the size of a tennis court—is home to about 111,000 spiders of the species Tegenaria domestica and Prinerigone vagans, according to a study by a separate group of researchers. This is the first documented case of either of these two species constructing webs together, the study’s authors reported recently in Subterranean Biology.

Sulfur caves are among the most extreme habitats on Earth. They are completely dark and filled with hydrogen sulfide gas, which is toxic to most life-forms. Inside, species’ survival depends on chemical reactions powered by microbes that oxidize sulfur. These microorganisms are the base of a unique food chain that supports a community of cave-dwelling organisms.

To understand how the spiders survive in this environment, the team analyzed the chemical signatures in their tissues, which revealed that the arachnids feed on tiny midges that hatch from cave pools. Those flies themselves depend on sulfur-oxidizing microbes as their primary food source.

A genetic analysis showed that the cave-dwelling spiders are becoming distinct from populations of the same species that live outside, suggesting they are adapting to the underground environment, the researchers wrote. They think that this genetic isolation, combined with a stable and abundant food supply, may have driven these species—which have never been reported to form colonies—to evolve colonial behavior.

The new finding “shows that nature still holds many surprises for us,” says study co-author Urák István of the Sapientia Hungarian University of Transylvania in Romania.