In the south-east of Romania, in Constanța county close to the Black Sea and the Bulgarian border, there lies a barren featureless plain. The desolate field is completely unremarkable, except for one thing.

Below it lies a cave that has remained isolated for 5.5 million years. While our ape-like ancestors were coming down from the trees and evolving into modern humans, the inhabitants of this cave were cut off from the rest of the planet.

Despite a complete absence of light and a poisonous atmosphere, the cave is crawling with life. There are unique spiders, scorpions, woodlice and centipedes, many never before seen by humans, and all of them owe their lives to a strange floating mat of bacteria.

In 1986, workers in communist Romania were testing the ground to see if it was suitable for a power plant, when they stumbled across the Movile Cave. Romanian scientist Cristian Lascu was the first to make the dangerous descent.

To enter, you must first be lowered by rope 20m down a narrow shaft dug into the ground

Since then the cave has remained sealed by the Romanian authorities. Fewer than 100 people have been allowed inside Movile, a number comparable to those who have been to the Moon.

This is partly because the journey into the cave is extremely hazardous.

To enter, you must first lower yourself by rope 20m down a narrow shaft dug into the ground. The only light is from your helmet, which bounces around the walls as you descend.

You must then climb down through narrow limestone tunnels coated in an ochre clay, in pitch darkness and temperatures of 25 °C. These paths eventually open out into a central cavern containing a lake.

In 2010, microbiologist Rich Boden, who was then at the University of Warwick in Coventry, UK, became roughly the 29th person to see the cave.

In the lake room, the atmosphere is heavy with harmful gases

"It's pretty warm, and very humid so it feels warmer than it is, and of course with a boiler suit and helmet on that doesn't help," says Boden, who is now at the University of Plymouth in the UK.

"The pool of warm, sulphidic water stinks of rotting eggs or burnt rubber when you disturb it as hydrogen sulphide is given off."

In the lake room, the atmosphere is heavy with harmful gases, principally carbon dioxide as well as the hydrogen sulphide from the water.

The experience is said to be terrifying – and that's even if you don't have a problem with creepy-crawlies

What's more, the air is low in oxygen: it contains just 10% oxygen rather than the usual 20%. Without breathing apparatus, you would soon develop a headache. Visitors can only stay down for 5 or 6 hours before their kidneys pack in.

To explore the rest of the cave, you must dive into the lake and navigate narrow underwater passageways, squeezing through tiny gaps in the rock before emerging into airspaces called air bells.

Doing this in complete darkness is the most dangerous part of exploring the cave. You are far from the surface, so getting stuck or losing your way in the maze of tunnels would be lethal. The experience is said to be terrifying – and that's even if you don't have a problem with creepy-crawlies.

Despite the dark and the dangerous gases, Movile Cave is crawling with life. So far 48 species have been identified, including 33 found nowhere else in the world.

The animals in Movile Cave seem to be without a source of food

There are all sorts of scuttling and slithering things. Snails and shrimps try to avoid the spiders and waterscorpions. In the air bells, leeches swim across the water and prey on earthworms.

Strangely, the worse the air gets the more animals there are. It's not at all obvious why that should be, or how the animals survive at all.

On the surface, plants use sunlight to extract carbon dioxide from the air and turn it into organic compounds. They can then use these chemicals to grow leaves, roots and bulbs. Animals then feed on these plant tissues.

Without sunlight, the animals in Movile Cave seem to be without a source of food.

In most caves, animals get their food from the water dripping down from the surface. This water can often be seen in the form of stalactites and stalagmites.

However, Movile Cave has a thick layer of clay above it, which is impermeable to water. When Lascu first visited, he could not find any stalactites or stalagmites, or any other sign of water coming from the surface.

The water in Movile Cave comes from a deep underground reservoir

The mystery deepened when scientists analysed the water in the cave for radioactive caesium and strontium. The 1986 nuclear accident at Chernobyl had released lots of these metals, which had found their way into the soils and lakes surrounding Movile Cave. However, a 1996 study found no traces of them inside the cave.

That means the water isn't coming from above, so it must be coming from below. It now seems that the water in Movile Cave comes from spongy sandstones where it has lain for 25,000 years.

However, this still doesn't explain how the animals in the cave survive. Tests have shown that the water flowing in does not contain any food particles.

Instead, the food comes from the strange frothy foam sitting on top of the water.

This floating film, which looks like wet tissue paper and can even be torn like paper, contains millions upon millions of bacteria known as "autotrophs".

Sulphuric acid actually erodes the limestone, which is gradually making the cave bigger

"These bacteria get their carbon from carbon dioxide just like plants do," says Boden. "The carbon dioxide level in the cave is about 100 times higher than normal air. But unlike plants, they obviously can't use photosynthesis as there is no light."

Rather than using light as an energy source, the Movile bacteria use a process known as chemosynthesis.

"They get the energy needed… from chemical reactions: the key ones being the oxidation of sulphide and similar sulphur ions into sulphuric acid, or the oxidation of ammonium found in the groundwaters to nitrate," says Boden.

These chemosynthetic bacteria help explain why the cave is so large and the air is so thick with carbon dioxide.

Movile is the only cave whose ecosystem is known to be supported in this way

"Sulphuric acid actually erodes the limestone, which is gradually making the cave bigger," says Boden. "The process releases carbon dioxide, which is why levels are so high."

Another major group of bacteria get their energy and carbon from the methane gas that bubbles up through the waters of the cave. They are called methanotrophs.

Boden describes methanotrophs as "messy eaters" that "constantly leak metabolic intermediates like methanol and formate" into the surrounding water. In turn, these chemicals are food for other species of bacteria.

This may all sound very peculiar, and in some ways it is. Movile is the only cave whose ecosystem is known to be supported in this way, and the only such ecosystem on land.

The Movile bacteria are very similar to bacteria found elsewhere

But according to microbiologist J. Colin Murrell of the University of East Anglia in Norwich, UK, the bacteria in Movile Cave are remarkably simple and not at all unusual.

"The bacteria get all of their carbon from just one source, be it methane or carbon dioxide," says Murrell. "That means that all of the components of their cells, be it the DNA in their nucleus, the lipids in their cell membrane and the proteins in their enzymes, are made from the same simple ingredient."

The Movile bacteria are also very similar to bacteria found elsewhere, despite having being trapped in the cave for over 5 million years.

"Methanotrophs are everywhere: the Roman Baths at Bath, the surface of seawater, the mouths of cattle and probably the human mouth and gut," says Boden. "Autotrophic bacteria of the same types we found at Movile are found in almost all soils and on the surface of the skin."

The same cannot be said for the animals of the cave. Millions of years of isolation has transformed them.

Many are born without eyes, which would be useless in the dark. Almost all are translucent as they have lost pigment in their skin. Many also have extra-long appendages such as antennae to help them feel their way around in the darkness.

One of the spiders was closely related to a spider found in the Canary Islands – which lie over 4000km to the west

There are no flies in Movile Cave, but the spiders still spin webs. Small insects called springtails bounce into the air and get caught in the webs.

In 1996, researchers categorised the animals in the cave. They included 3 species of spider, a centipede, 4 species of isopod (the group that includes woodlice), a leech never seen anywhere else in the world, and an unusual-looking insect called a waterscorpion.

Strangely, one of the spiders was closely related to a spider found in the Canary Islands – which lie over 4000km to the west, off the north-west coast of Africa.

That raises the question, how and why did the animals get into the cave?

One theory is that back at the end of the Miocene Epoch, about 5.5 million years ago, the climate of the northern hemisphere changed. As Africa moved north it stopped the Atlantic from flowing into the Mediterranean Sea, drying it out.

It's very likely that the bacteria have been there a lot longer than five million years

This could have forced the animals to seek refuge in the sulphurous underworld of Movile Cave. It would have been a haven, with thermal waters providing constant warmth, no competitors or predators, and a rich source of food.

The problem with this theory is that it is difficult to prove.

"It's very likely that the bacteria have been there a lot longer than five million years, but that the insects became trapped there around that time," says Murrell. "They could have simply fallen in and become trapped when the limestone cast dropped, sealing the cave until it was discovered again in 1986."

It may be that different animals arrived at different times. A 2008 study of Movile's only snail suggested that it has been down there for just over 2 million years. When it entered the cave, the ice age was just beginning, and the snail may have escaped the cold by going underground.

However they got there, it seems that Movile's inhabitants are now trapped for good. We could learn a lot from them.

It could be that the first living cells were similar to those found in Movile Cave

The bacteria's ability to oxidise methane and carbon dioxide is of particular interest. These two greenhouse gases are the biggest culprits for global warming, so researchers are desperate to find efficient ways to remove them from the atmosphere.

The Movile Cave microbes could also offer hints about how the first life formed on Earth. They are genetically similar to those found in geothermal vents, which are also rich in carbon dioxide, sulphides and ammonia.

The conditions in both places may well be similar to the primordial Earth. In our world's early years, the Sun's light was obscured by an atmosphere thick with carbon dioxide, methane and ammonia. It could be that the first living cells were similar to those found in Movile Cave.

Almost 30 years after its discovery, Movile Cave remains perhaps the most isolated ecosystem on the planet. It surely has many more secrets to give up. There are plenty more organisms buried in the cave's sediments, waiting to be identified, and they could help us understand some of our deepest questions about the nature of life.