In 1986, workers were drilling an exploratory borehole near the city of Mangalia, on Romania’s Black Sea Coast. They were looking for somewhere to build a power plant, but instead, they stumbled across an underground cave system that had been cut off from the rest of the world for over 5 million years.
The Movile Cave is a huge, maze-like system of tunnels, buried under 18 metres of clay and limestone. This is a dark and eerie place. The air is heavy with toxic gases. What little water there is, is low in oxygen and rich in chemicals, such as hydrogen sulphide, methane and ammonia. It’s murky and tepid, acrid and pungent. Most organisms would struggle to survive there, and yet, somehow, life doesn’t just survive. It positively thrives.
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The Movile Cave is home to a rich and unique ecosystem of self-sustaining organisms. More than 50 species of invertebrate have been found here, of which 37 live nowhere else.
This includes spiders, water scorpions, worms, beetles and a newly discovered species of centipede dubbed ‘king of the cave’ in a 2020 scientific paper.
Sealed off from the world above, with no trace of sunlight, these animals have evolved specialist adaptations to life in the dark. Many of them have no eyes or skin pigmentation.
Some have extra-long limbs and antennae to help feel their way around. The ‘king of the cave’ has saw-like serrations on its last pair of legs, which are thought to help it to find and then grapple its prey. So, how do these organisms survive?
Above ground, life is powered by photosynthesis. Plants and algae use sunlight to turn carbon dioxide and water into energy and oxygen. In the Movile Cave, that’s not possible. So, instead, life is powered by chemicals. This is known as chemosynthesis.
Bacteria form base of the food chain. They coat the walls of the cave and sit in a thick layer of scum on the surface of the cave’s underground lake and puddles. These microbes oxidise hydrogen sulphide, methane and ammonia, converting inorganic molecules into organic ones that other organisms in the cave can use. Small invertebrates, such as snails and nematode worms, eat the bacteria, and bigger organisms, such as centipedes, leeches and spiders, eat them.
The bacteria even help to shape the cave. When they oxidise hydrogen sulphide, they produce sulphuric acid, which slowly eats away at the limestone walls. Over time, this enlarges the cave, which now sprawls over 240 metres, and releases more carbon dioxide from the limestone. Some of the bacteria also make carbon dioxide, and the air in the cave contains about 100 times more of the gas than the air above ground.
Today, the entrance to the bore hole has been sealed shut, and entry to the cave is severely limited. Only a handful of researchers are permitted to visit each year, for short durations only, with specialist equipment.
The restrictions are there to protect the health of those who visit, but equally important, they help to minimise contamination and preserve the delicate balance of this truly remarkable ecosystem.
Top image: Various aquatic invertebrates from Movile Cave, including Haemopis caeca (A), Asellus aquaticus infernus ; (C). (Pseudocandona sp. ; (D).(Nepa anophthalma); (E) Heleobia dobrogica). © Traian Brad, Sanda Iepure and Serban M. Sarbu, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
