Movile Cave

Movile Cave () is a cave near Mangalia, Constanța County, Romania discovered in 1986 by Cristian Lascu a few kilometers from the Black Sea coast. It is notable for its unique groundwater ecosystem abundant in hydrogen sulfide and carbon dioxide, but low in oxygen. 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.
Chemical environment.
The air in the cave is very different from the outer atmosphere. The level of oxygen is only a third to half of the concentration found in open air (7–10% O2 in the cave atmosphere, compared to 21% O2 in air), and about one hundred times more carbon dioxide (2–3.5% CO2 in the cave atmosphere, versus 0.04% CO2 in air). It also contains 1–2% methane (CH4) and both the air and waters of the cave contain high concentrations of hydrogen sulfide (H2S) and ammonia (NH3).
Biology.
The cave is known to contain 69 animal species, among them leeches, spiders, pseudoscorpions, 
woodlice,
a centipede,
a water scorpion (insecta),
and also a snail. 
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. One of the most recent animals recorded is the cave's only species of snail, "Heleobia dobrogica", which has inhabited the cave for slightly more than 2 million years.