Discovered in 1986, Movile Cave in Romania "is notable for its unique groundwater ecosystem abundant in hydrogen sulfide and carbon dioxide, but low in oxygen."
The cave's oxygen level "is only a third to half of the concentration found in open air..., and [the cave has] about one hundred times more carbon dioxide. It also contains 1–2% methane .. and both the air and waters of the cave contain high concentrations of hydrogen sulfide and ammonia.
Life in the cave has been separated from the outside for the past 5.5 million years and it is based completely on chemosynthesis rather than photosynthesis.
"The cave is known to contain 48 species, among them leeches, spiders and a water scorpion. Of these, 33 are endemic. The food chain is based on chemosynthesis in the form of methane- and sulfur-oxidizing bacteria, which in turn release nutrients for fungi and other bacteria. This forms microbial mats on the cave walls and the surface of lakes and ponds which are grazed on by some of the animals. The grazers are then preyed on by predatory species. Nepa anophthalma is the only known cave-adapted water scorpion in the world. While animals have lived in the cave for 5.5 million years, not all of them arrived simultaneously. The most recent animal recorded is the cave's only species of snail, which has inhabited the cave for slightly more than 2 million years." -- https://en.wikipedia.org/wiki/Movile_Cave
Good article here: http://www.bbc.co.uk/earth/story/201...as-poison-cave