[SciShow intro plays]

Hank: Weird stuff happens in caves. Venture into the darkness and you never know what you might find: glow worms, crystals, millions of sleeping bats, a Horcrux, Grendel's mom...

In 1986, a prospecting crew in southern Romania was looking for a good place to build a geothermal power plant, when they accidentally discovered one of the oddest caves of all. It's called Movile cave, and it's basically a time capsule, crawling with strange life that has lived separate from the rest of Earth for millions of years.

More than 20 meters underground, the cave is pitch dark, warm, and full of toxic gases, and holds a lake that reeks of burnt rubber and rotten eggs. And other than the prospecting shaft, the cave is totally sealed up in all directions. You heard me right, it's been cut off from the rest of the world for about five and a half million years, making it one of the most isolated places on Earth.

Thanks to the thick layers of clay above the cave, nothing seeps down from the Earth's surface: no sunlight, no food, and no water. There aren't even any traces of radioactive metals from the Chernobyl nuclear accident, which are spread throughout the soils and lakes near the cave. But there are all sorts of hardcore creepy crawlies down there, and they've formed a complex ecosystem.

Movile's lake water is covered by a tissue paper-like mat made up of billions of autotrophic bacteria, which can turn inorganic compounds like carbon dioxide, along with some energy, into food. And the air in the cave has about 100 times more carbon dioxide than outside air, which provides them all the CO2 they need.

Here on Earth's surface, lots of organisms, like plants, use light and carbon dioxide to make food. That's photosynthesis. But these bacteria cannot use photosynthesis, they live in total darkness, so they don't have access to sunlight or any light energy at all. So instead they use chemosynthesis, or energy from chemical reactions involving the toxic gases in the cave, like hydrogen sulfide, to help generate their food.

Other kinds of bacteria floating in the lake use methane gas as their only source of carbon and energy. Worms, shrimp, and other smaller organisms feast on all of these microbes, and then larger invertebrates, like spiders and centipedes, feed on them.

Not only do these chemosynthetic bacteria support an entire ecosystem, but they also end up shaping the cave itself. Reactions involving hydrogen sulfide gas produce sulfuric acid, which erodes the limestone walls over time. That gradually makes the cave bigger and also releases more carbon dioxide gas. It's a pretty cool ecosystem and the Movile Cave is the only place we know like this.

At least, on land. Deep down in the ocean there are geothermal vents thick with the same types of toxic gases found in the cave, and there are similar chemosynthetic bacteria living around those vents. Both Movile Cave and the vents might resemble the dark and gassy conditions of the early days of planet Earth, which has left some scientists wondering if these Movile microbes might be similar to the first kinds of life.

But even though similar bacteria have been found elsewhere, what about all those larger invertebrates? To be honest, they're a bunch of creepy looking weirdos, but I guess living in total isolation and darkness for millions of years will do that to you. Scientists have discovered 48 species in the cave, 33 of which were previously unknown, including three spiders, a leech, a centipede and a very unusual insect called a water scorpion that actually seems to be related to bed bugs.

These organisms show the troglomorphic adaptations that are associated with life in the dark, like lack of skin pigmentation, and extra long antennae to help them feel around in the darkness. In fact many are born without eyes at all, since they don't have much use for them.

By now though you might be wondering how the heck these creatures got stuck underground in the first place. Well, we're not entirely sure. One hypothesis is that the changing climate at the end of the Miocene five and a half million years ago may have had something to do with it. Colder, drier conditions on the Earth's surface could have sent surface invertebrates underground, looking for shelter and warmth. And when the cave eventually sealed up, they were trapped.

One of Movile's spiders is closely related to a surface species found four thousand kilometers away in the Canary Islands which may support this theory. Then again, it's also possible that some critters just fell in through cracks in the limestone more recently, like two million years ago instead of five. Either way it's a fascinating look at the strange ecosystems that can form in extreme places.

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