Movile Cave is a cave near Mangalia, Constanța County, Romania discovered in 1986 by Cristian Lascu during construction work a few kilometers from the Black Sea coast.

It is notable for its unique subterranean 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. Due to its extreme environment, access to Movile Cave is strictly controlled, and a limited number of researchers have permission to study its conditions. 

The air in the cave is extremely 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 100 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). The lake waters only contain as little as 1 mm of dissolved O2, and up to 1 cm of dissolved O2 at most. As depth increases in the lake, the water becomes completely anoxic. 

Movile Cave supports a complex ecosystem built upon chemosynthetic bacteria. These bacteria derive energy from oxidizing hydrogen sulfide and methane, forming the base of the food web. This makes the cave one of the few known ecosystems that do not rely on sunlight as a source of primary production.

The organisms within Movile Cave have gained unique adaptations in order to survive its extreme chemosynthetic environment. Many of these species lack functional eyes, as vision is not necessary in complete darkness. As a replacement, many species develop enhanced mechanosensory and chemosensory awareness that enables species to detect food and move in the darkness.

Due to the high concentration of toxic gases, many organisms have developed physiological mechanisms in order to tolerate the elevated carbon dioxide and hydrogen sulfide levels. Some species exhibit specialized respiratory adaptations that increase the efficiency of oxygen extraction from the cave's low oxygen atmosphere.

These features not only better our understanding of life in extreme environments on earth, but also have provide insight into the possibility of microbial life in extraterrestrial subsurface environments. 

The long-term isolation of Movile Cave has led to unique evolutionary adaptations of its microorganisms and eukaryotic communities. That means they have undergone genetic divergence in small isolated populations and established novel symbioses, such as between invertebrates and sulphur-oxidizing bacteria. Studying these systems provides insights into the early evolution of eukaryotes with an interest in protists adapted to anoxic conditions. In this respect, Movile Cave also provides an excellent terrestrial analogue for deep-sea hydrothermal vents and extraterrestrial ecosystems, favouring the scenario of life existing subsurface beneath moons like Europa or Enceladus.

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