(CNN) — Postojna Cave, situated one hour's drive southwest of Ljubljana, Slovenia's capital, is so vast, it has its own railway.
And yet, one of the cave's main attractions is something at the other end of the size spectrum -- and completely unique to Postojna.
Postojna is, during normal travel times, one of Europe's most visited underground attractions.
It's been known to locals for centuries and has the graffiti, dated 1213 to prove it. Tourists began arriving in serious numbers after an inaugural 1818 visit by Franz I of Austria, Europe's last Holy Roman Emperor. About 35 million have followed in his wake.
It's easy to see why. The cave is so big that a small train runs for the first two of its 24 kilometer network of subterranean chambers and tunnels.
The train line ends at the massive Congress Hall where the Milan Symphony Orchestra performed back in 1930. From there a walking path goes through six geological strata, crosses a bridge over a chasm built by Russian prisoners during World War I and continues past underground clifftops and gorges, spaghetti-thin stalactites and flowstone curtains.
Journeying to depths of 115 meters (377 feet), at times it takes visitors through slits just one meter wide.
And yet the real adrenaline rush is saved for coming face to face with the kooky creatures that are found in the Postojna cave system and nowhere else on Earth.
Olms grow up to 25 centimeters in length.
Courtesy Postojna Cave D. D
Olms, or proteus anguinus in Latin, are blind salamanders, about 25 centimeters long, that never develop beyond their juvenile, watery phase.
Locals dubbed them baby dragons because they were washed out of Postojna during floods and, as caves are the abode of dragons, surely these were their babies, right?
Nowadays, visitors can encounter them swimming among the rocks in a purpose-built aquarium deep within the cave.
"Cute aren't they?" asks Mateja Rosa, a big olm fan, who works as Postojna's marketing and PR manager.
They are indeed. Almost toylike in appearance, they're also sometimes called human fish because -- despite living underwater -- they have pinkish-white, smooth skin instead of scales, and limbs with cartoon-like fingers below their busy red gills.
They may be blind, but the olms seem to hear the approach of visitors, apparently sensitive to vibrations. One even attaches itself to the glass tank near to where my face is peering in.
Is it curious? Is it being friendly?
Not so, according to Primož Gnezda, a young, enthusiastic biologist who's been studying these creatures for years.
"The olms in the cave tank hear you, get frightened and assume their safe positions," say Gnezda during a tour of the Vivarium, an exhibition space next to the cave that displays more olms and a slew of other Postojna creatures.
The apparently friendly olm is known for its unusual behavior, but wasn't being gregarious.
"It always spreads against the glass for safety," says Gnezda. "That it appeared next to your face was just a coincidence."
Visitors to Postojna can see olms in an aquarium.
Jure Makovec/AFP/Getty Images
According to Rosa, olms can live to 100 years and can survive long periods without eating.
"Seven years for sure," she says. "For the first two - three years, no problem. Afterward they start to lose weight, stop moving and simply wait for prey to pass by. Any longer than seven years and some may die, some may survive, depending on the individual's metabolism."
When they do find food, we can forgive their manners.
"We feed them worms," says Gnezda. "The worms form a small ball together in the water and the olms come and hoover it up whole like a vacuum cleaner. Sometimes they eat so violently that you can see worms coming out of their gills along with the water."
The Vivarium leads to the lab where scientists have a license to keep 10 olms for research. A lot of money is spent on these creatures.
"Biologists have been researching their DNA," says Gnezda. "Their genome is like a novella. It's 16 times longer than the human one and more complicated."
"You also have a lot of empty spaces. We don't know why they exist. Imagine a book 600 pages long, where all the words are scrambled and we must reconstruct the story."
Is there a reason we're so interested?
"Their regeneration power is amazing. If they lose a limb, they regrow it. The idea of the research is to find out the mechanism behind it."
"Not to actually grow back your arm or your leg, but maybe to produce a new human hand or a leg from your own cells inside a laboratory and then graft it onto you. That's, of course, far, far into the future."
The olms can live up to 100 years.
Jure Makovec/AFP/Getty Images
Given that the olms are cute, don't require feeding and they will probably outlive you, Rosa says that in the past they were sometimes given as pets to visiting dignitaries.
"Most died," she adds. "Olms must be kept at around 13 Celsius (55 F). If the temperature rises quickly, say from 10C to 15C, it kills them."
Salamanders start their lives in the water like olms but then they drop their gills, develop lungs, walk on land and mature sexually; yet olms remain and multiply in the juvenile stage -- a biological oddity like their close relative the axolotl, also dubbed the Mexican walking fish.
Olms even have a mating dance.
"It goes like this," Gnezda says. "When the female is ready, she'll come to the male. When he smells her, he'll start swimming in front of her; she'll follow him and do a few circles together.
"At one point the male will leave a packet of semen on the floor. She'll pick it up and store it in a pocket inside her. When an egg comes out, it'll be fertilized on its own."
And that's not all.
"You can't tell whether an olm is male or female from its DNA. Both male and female have the same chromosomes. Now we're trying to distinguish between sexes by analyzing their blood and checking the hormone ratios. It looks promising but this is still ongoing research."
Biologist Primož Gnezda is among scientists studying the olms.
Jure Makovec/AFP/Getty Images
Now to the big announcement.
On January 30, 2016 one female started feeling very territorial and attacked the other olms if they approached her; to the delight of the researchers, they saw that she was guarding an egg.
Her companions were immediately removed and her tank was isolated. Infrared cameras revealed that she kept laying eggs for another eight weeks.
"She eventually produced 64 eggs," Gnezda says. "In nature the mother sticks the eggs on rocks, as there's no real predator out there in the cave."
"But a lot can go wrong while the egg is developing and around two thirds of the young just die by themselves."
Exactly four months after the first egg had been laid, the first baby baby dragon hatched. It thrust out, fell onto the bottom of the aquarium and then swam around precociously.
All in all 21 survived. Intriguingly, they're born with eyes which they keep for several years until skin grows over them and renders them blind.
And since June 2021 two of those five-year-old olms are now on display.
As Gnezda reveals during a tour of the Vivarium, they're not Postojna's only unusual occupant.
There are cave crickets that eat their own limbs if they can't find any food; poisonous cave millipedes; slenderneck beetles whose wings have atrophied and fused on their abdomen; cave shrimps, the olms' preferred snack; and the obligatory bloodcurdling spider -- since there are no flying insects inside the cave, spiders use their silk to weave cocoons rather than webs.
Talking of food, when the olms were flushed out to rivers by floods, did they ever wind up on someone's dinner plate?
Yes, says Rosa. "Up until the 1980s you could find them sold on the slab in the fish markets in Trieste."
"They taste like bland calamari. Or so I'm told."