The Spider Megacity: The Massive Web Scientists Found in Sulfur Cave

Imagine stumbling into a cave and finding yourself face-to-face with a spider web the size of half a tennis court. Now imagine that web is home to more than 111,000 spiders living together in total darkness, in an environment filled with toxic gas that would kill most living things. Nightmare fuel? Sure. But also one of the most fascinating ecological discoveries in recent years.

Welcome to Sulfur Cave, straddling the border between Albania and Greece, where scientists have discovered what may be the largest spider web ever recorded.

The Discovery: A Web Like No Other

In 2022, cavers from the Czech Speleological Society were exploring Vromoner Canyon (which literally means "smelly water" in Greek) when they stumbled upon something extraordinary. Deep inside Sulfur Cave, in a narrow, low-ceilinged passage that never sees sunlight, they found a massive colonial spider web stretching across the cave wall.

The web spans 1,140 square feet and is made up of thousands of individual, funnel-shaped webs all connected together. It starts about 164 feet from the cave entrance and extends through a dark zone where no natural light ever reaches.

The cavers knew they'd found something special and alerted scientists, who made several expeditions to study the phenomenon. What they found was extraordinary: approximately 69,000 domestic house spiders (Tegenaria domestica) and more than 42,000 sheetweb spiders (Prinerigone vagans) living together in what researchers are calling an "arachnid megacity."

Why This Is So Unusual

Here's the thing that makes this discovery truly wild: these spider species don't normally do this.

Both Tegenaria domestica and Prinerigone vagans are usually solitary spiders commonly found near human dwellings, but this is the first documented case of colonial web formation for either species. They're the kind of spiders you might find in your basement or garage, minding their own business, building their own individual webs. They definitely don't share.

Even more surprising, you'd normally expect the larger house spiders to prey on the smaller sheetweb spiders. But scientists think the lack of light in the cave may impair the spiders' vision, preventing this typical predator-prey relationship. In the pitch-black environment, cooperation has apparently replaced competition.

Lead researcher István Urák, a biologist at Sapientia Hungarian University of Transylvania, described seeing the web for the first time as an overwhelming experience. "You have to experience it to truly know what it feels like," he said. The sheer scale and density of the spider colony is unlike anything documented before.

The Cave: An Alien Environment

Sulfur Cave isn't your typical tourist-friendly cavern. It's one of the most extreme habitats on Earth.

The cave was carved out by sulfuric acid formed from hydrogen sulfide in groundwater. A sulfur-rich stream fed by natural springs flows through it, filling the air with hydrogen sulfide gas, which is toxic to most life forms. The cave is completely dark, with temperatures hovering around 79°F year-round. It's permanently damp, smells terrible (hence "Vromoner"), and would be deadly to most creatures.

But for the spiders, it's home.

The Food Web: Life Without Sunlight

Most ecosystems on Earth ultimately depend on the sun. Plants use photosynthesis, herbivores eat plants, carnivores eat herbivores, and so on. But Sulfur Cave operates on an entirely different system called chemosynthesis, where life is powered by chemical reactions instead of sunlight.

Here's how the Sulfur Cave food chain works:

1. Sulfur-oxidizing bacteria are the foundation. These microbes feast on chemicals in the cave and form white microbial biofilms (slimy coatings) on the cave walls.

2. Non-biting midges hatch from pools in the cave and feed on these microbial mats. Researchers found an "unusually dense" swarm of small midge flies called Tanytarsus albisutus in the cavern.

3. Spiders catch the midges in their webs. The midges provide an abundant, continuous food source that never runs out.

Using stable isotope analysis, researchers confirmed the spiders weren't eating insects from outside the cave but were fully integrated into this chemosynthetic food web. The entire ecosystem is sustained by chemical reactions that would poison most other life.

They're Evolving Into Something New

Perhaps the most fascinating finding is that the cave-dwelling spiders are genetically distinct from populations of the same species living just outside the cave. They're adapting to their underground environment in real time.

Analysis of the spiders' gut microbiomes revealed they are significantly less diverse than their surface-dwelling relatives, likely because of their specialized sulfur-rich diet. The spiders are eating food that's fundamentally different from what their cousins eat above ground, and it's changing them at a biological level.

This genetic isolation, combined with the stable food supply and unique environmental pressures, may be driving the evolution of colonial behavior in species that have never shown it before. The cave is essentially creating new behavioral adaptations in these spiders.

The Numbers Are Staggering

Let's put the scale of this colony in perspective:

• 111,000+ spiders living in one interconnected web structure

• 1,140 square feet of web coverage (about half a tennis court)

• Two different species peacefully coexisting

• Complete darkness with no natural light ever reaching the colony

• First documented colonial behavior for both species involved

For comparison, most spider colonies (when they do occur) involve a few hundred to maybe a few thousand individuals. This is orders of magnitude larger.

Other Massive Spider Webs

While the Sulfur Cave web may be the largest by area, it's not the only impressive spider web in the world.

Darwin's bark spider, found in Madagascar, builds individual orb webs that can span up to 82 feet across rivers and streams. A single spider, smaller than a quarter, can create a web large enough to catch 30 or more insects at once. These webs are made from some of the toughest biological material known, about ten times stronger than Kevlar.

But what makes the Sulfur Cave web unique isn't just its size. It's the cooperation, the number of individuals involved, and the fact that it exists in such an extreme environment sustained by chemosynthesis rather than photosynthesis.

Why This Matters

Beyond being absolutely wild to think about, this discovery has real scientific significance.

It shows nature's adaptability. Life finds a way, even in toxic, dark, sulfur-filled caves where most organisms would die immediately. The spiders have adapted to an environment that seems completely inhospitable.

It reveals new behaviors. The colonial living and peaceful coexistence between two species that don't normally cooperate suggest that environmental pressures can drive rapid behavioral evolution.

It highlights genetic plasticity. The genetic differences between cave and surface populations show how quickly species can adapt when isolated in extreme conditions. Urák noted that "some species exhibit remarkable genetic plasticity, which typically becomes apparent only under extreme conditions."

It demonstrates the importance of cave ecosystems. Caves are often overlooked in scientific research, but they can harbor unique species and ecological relationships found nowhere else on Earth. Sulfur Cave connects to two other caverns (Atmos Cave and Turtle Cave), creating an underground maze that likely holds even more surprises.

The Challenge of Conservation

There's a complication: the cave straddles two countries. Its entrance is in Greece, but its deepest sections extend into Albania. This creates challenges for conservation efforts, as protecting the site requires cooperation between two different governments with different regulations.

Urák emphasized the importance of preserving the colony despite these challenges. The spider megacity is a unique natural phenomenon that deserves protection, but international coordination will be necessary to ensure it survives.

Human activity, pollution, climate change, and even increased visitation from curious researchers could potentially disrupt the delicate ecosystem. The entire food web depends on the chemical balance in the cave, and any changes to water chemistry or sulfur levels could have cascading effects.

The Bigger Picture

The Sulfur Cave spider colony reminds us that Earth still has plenty of secrets. Even in 2024, with satellites mapping every inch of the planet's surface, we're still discovering entirely new ecosystems and behaviors in places we thought we understood.

Caves, in particular, remain largely unexplored. According to Patricia Kambesis, director of the American Cave Conservation Association, caves "have not generally attracted the attention of mainstream scientists" despite their potential value. The Sulfur Cave discovery proves that systematic cave exploration can yield remarkable findings.

There are thousands of unexplored caves around the world. How many other spider megacities are out there? What other extreme adaptations are happening in the dark, in environments we've barely begun to study?

The Takeaway

More than 111,000 spiders living together in complete darkness, sustained by a food web powered by toxic chemicals, exhibiting behaviors their species have never shown before, adapting genetically to their environment in real time. It sounds like science fiction, but it's happening right now in a cave on the Albania-Greece border.

The next time you see a house spider in your basement, remember: its cousins might be part of the largest spider colony ever documented, thriving in conditions that would kill you in minutes, building webs that span hundreds of square feet, and possibly evolving into something new.

Nature is weird, incredible, and still full of surprises. And sometimes, the most amazing discoveries are hiding in the darkest, most inhospitable places we can imagine.

Sources

Urák, I., et al. (2025). An extraordinary colonial spider community in Sulfur Cave (Albania/Greece) sustained by chemoautotrophy. Subterranean Biology, 53, 155-177. https://doi.org/10.3897/subtbiol.53.162344

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