### Movile Cave
Romania — Sealed off from the sun for 5.5 million years, this toxic cave is crawling with blind, pale insects.
Discovered by accident during construction in 1986, Movile Cave is an underground world that has been completely sealed off from sunlight for 5.5 million years. The air inside is deadly to humans, packed with carbon dioxide, methane, and toxic hydrogen sulfide gas. Because there is absolutely no sun, there is no photosynthesis. Instead, the cave's food web relies on a process called chemosynthesis. Specialized bacteria literally eat the toxic, rotting-egg-scented gases to create energy, forming a thick, slimy microbial mat on the walls.
This toxic slime is an all-you-can-eat buffet for 48 distinct species of pale, completely blind cave animals—including spiders, leeches, and water scorpions—33 of which exist nowhere else on Earth. But while scientists understand how they survive, they are still fiercely debating when they arrived.
Did these creepy-crawlies get locked inside 5.5 million years ago when the Mediterranean Sea dried out during the Miocene epoch? Or did some of them squeeze in much later—say, 2 million years ago—through microscopic, undiscovered cracks in the limestone before the cave was fully sealed off? Evolutionary biologists are still studying the DNA of these bizarre critters to solve the mystery.