There aren’t any good pictures of the inside of this place (for potentially very good reason!), which I found disappointing. I really wanted to share one here! — Dan

Spiders, insects, and worms that have never seen sunlight. A predatory centipede that feasts in the darkness. A huge microbial soup, with bacteria floating on thick mats across its surface. Air that is toxic for humans to breathe. It sounds like something from Stranger Things, but this isn’t the Upside Down. It’s a place in Romania, not too far off the coast of the Black Sea. And the stuff inside thrives, despite ignoring the normal rules of life.

Welcome to Movile Cave, the place you almost certainly do not want to visit.

In 1986, workers in southeastern Romania were conducting geological surveys for a potential power plant site when they drilled into a previously unknown underground cavity. When researchers investigated the shaft, they realized they had found something extraordinary: a cave system sealed off from the outside world for millions of years. Being literally walled off from the rest of the planet, it developed a strange ecosystem — toxic air, sulfur-rich water, and dozens of strange organisms adapted to life without sunlight. And yet, inside, there was life, virtually undisturbed for more than five million years.

In caves, the usual story is that life survives off whatever drips or washes in from the surface — crumbs from the sunlit world above. Movile, though, doesn’t work that way. It is cut off from the rest of us entirely, and its food chain is built around a floating mat of microbes — basically, a slimy heap of goo that doesn’t need sunlight. Instead of photosynthesis, this ecosystem runs on chemosynthesis — microbes turning chemicals into usable energy. The cave is powered less like a forest and more like a chemistry lab.

Scientists have explored the cave ever since its accidental discovery — and few others have entered it. As the BBC reported in 2015, “fewer than 100 people have been allowed inside Movile, a number comparable to those who have been to the Moon.” That’s because local authorities want to keep the site and its ecology as undisturbed as possible, and because it’s not a very safe place to be. The harmful, low-oxygen air and pitch darkness aside, it’s a difficult trip to get inside — per the BBC, “to enter, you must first lower yourself by rope 20m down a narrow shaft dug into the ground” and then climb through narrow tunnels that weren’t created with human explorers in mind.

But if you’re fortunate (and brave) enough to make the expedition, you’ll likely find something few others have ever seen. In 1995, the New York Times reported that scientists discovered 48 different species of life in the cave, 33 of which were unknown to us beforehand — creatures who live nowhere else on the planet (as far as we know), and had thrived in this crypt for millennia if not longer. And that doesn’t include that centipede — it wasn’t discovered until 2020. But now, per Discover Wildlife, it’s considered the “king of the cave,” in no small part because it appears to be at the top of Movile’s eating hierarchy — little creatures eat microbial mats, bigger creatures eat the little creatures, and the 2-inch (5 cm) centipede dines on them.

For millions of years, Movile Cave carried on in complete darkness, powered by toxic gases and crawling with creatures that had never seen the sun. Humanity only discovered it by accident — which makes you wonder what other strange little worlds might still be hiding underground.

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Today’s Bonus fact: The Movile Cave centipede is big for a centipede — most species are half its typical length. But one other cave-dwelling centipede is much, much bigger. The Peruvian giant yellow-leg centipede is the world’s longest, stretching to a foot (30 cm) in length. Being that big, it also has an expanded diet, eating snakes, mice, small birds, and bats — particularly when found in caves. Per its Wikipedia entry, larger versions of the centipede “have been known to employ unique strategies to catch bats with muscular strength. They climb cave ceilings and hold or manipulate their heavier prey with only a few legs attached to the ceiling.” (Sorry if that gives you nightmares!)

From the Archives: The World’s Largest Cave: It’s huge.

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And thanks! — Dan