Last updated at 22:16 15 July 2007

Some locals say they are eight feet tall and weigh 20 stone. Others claim they are even bigger, equipped with huge flesh-ripping teeth and muscles capable of dismembering a man.

Forest dwellers have told visiting explorers and scientists of a ferocious grey ape, with the cunning of a chimpanzee and the power and size of a gorilla - and a taste for meat rather than the shrubbery loved by most apes.

For, unlike most apes, these are predators - capable of hunting not only forest antelope but, incredibly, lion and leopard too. And to cap it all, like wolves, these fearsome beasts howl at the Moon.

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The giant lion-eating ape of the Congo, known 'officially' as the Bili or Bondo Mystery Ape, is one of those creatures that for decades has sat on the knife-edge that divides myth and scientific respectability.

Sceptics say giant, lion-eating primates are no more than a fairy tale, a by-product of the sheer size and remoteness of Africa's largest, most lawless, unexplored and war-torn region, into which any number of fictional monsters can be placed by an overactive imagination.

Indeed, the Congolese super-ape, a grey-furred, ferocious man-eating carnivore, starred in Michael Crichton's 1980 sci-fi thriller Congo, and cryptozoologists (people who hunt for mythological beasts) also talk of Mokele-mbembe, Africa's Loch Ness monster, a pink dinosaur purported to survive in the Congolese forests.

And yet, despite the aura of mythology that surrounds these improbable creatures, many scientists believe that something new and unusual does indeed live in the impenetrable forests of Africa's Heart of Darkness.

In particular, the locals' tales about the giant, unusual apes which have been noted by Western explorers since the late 1890s are simply too similar, too coherent, to be dismissed as fanciful exaggeration.

And now, it seems that Congolese mystery ape is finally coming into the spotlight of scientific respectability.

At last, a group of researchers has succeeded in studying these animals first-hand over a long period, and has not only confirmed their existence, but also described a fascinating animal about whom not all the myths are true, and yet which lives up to - in some respects - its legendary reputation.

Congolese people have long told Western explorers and biologists about a species of ape which looks like a big chimpanzee, yet which sleeps on the ground like a gorilla and hunts big cats for food.

A Swiss photographer called Karl Amman visited the area in 1996 and found a skull which was similar in size to that of a chimpanzee and had a prominent bony crest like a gorilla. Could this be some sort of gorilla chimp hybrid?

Or was it perhaps a completely new species? There is, after all, probably no genetic reason why gorillas and chimps (which do occasionally meet in the Congolese forests) could not produce viable offspring.

Amman got hold of a photograph, taken by bush meat hunters, which appeared to show a huge chimpanzee. He found droppings several times larger than chimp dung and footprints bigger than a gorilla's.

He recounted what locals had told him about the animals. "Gorilla males will always charge when they encounter a hunter, and if you were charged by a gorilla you would never forget it, but there were no stories like that," Amman says.

Instead, these apes would come face-to-face with their human cousins, stare intently, then slide away quietly. No aggression, yet no fear either.

Then, in 2004, Shelly Williams, a primatologist affiliated to the Jane Goodall Institute, revealed the first recorded close encounter by a scientist with these creatures, in the New Scientist.

"We could hear them in the trees, about 20ft away - and four suddenly came rushing through the bush towards me," she wrote. "If this had been a bluff charge they would have been screaming to intimidate us.

"These guys were quiet, and they were huge." At first, she feared they were 'coming in for the kill', but perhaps sensing an unknown danger, they thought better of it and retreated.

She said the apes had a flat face, with a wide muzzle and - most strikingly - grey fur all over their face and bodies. It seems that Crichton's fictional grey killer apes had been found.

The report was met with some scepticism, but most scientists were convinced that something new had been discovered.

After all, it was in the forests of Central Africa that one of the most startling discoveries of modern zoology was made.

When sightings of a massive primate living high in the cool mists which drape the volcanic peaks of what are now Rwanda, Uganda and the Congo were mooted in the late 19th century, few took them seriously.

Explorers described a powerful yet gentle, celery-munching ape living at altitudes of more than 10,000ft. But it wasn't until 1902 that Robert von Beringe, a German army officer, made detailed observations of the animal which now bears his name, Gorilla gorilla beringei, the famed mountain gorilla.

Now, a researcher called Cleve Hicks at the University of Amsterdam has spent 18 months observing the Bili apes at close quarters. What he found was intriguing - a population of extremely large chimpanzees with their own distinct 'culture' and, indeed, a liking for the meat of big cats.

One was seen scavenging on the carcass of a leopard, although it is not known if the ape had killed the cat.

"The adult males do appear to be larger than the average chimpanzee male," Hicks writes in his latest report. "Although I doubt nearly as large as gorilla males."

Perhaps the main difference between these animals and other chimpanzees is their fearlessness of humans. Most chimps instinctively flee when they catch sight of a man; these animals approached the scientists and their staff with a degree of curiosity.

Other differences include the fact that these chimps live predominantly on the ground, like gorillas, in large nests of twigs and foliage.

Why they do so is unclear; most chimps like to sleep high in the trees, safe from predators. Down on the forest floor these animals could be easy prey for prowling night hunters such as leopards.

Hicks speculates that maybe the leopards have a healthy respect for these apes, knowing full well what they are capable of, and leave them alone.

So how does the reality match up to the myth? A new species or gorilla-chimp hybrid? Probably not; all the indications are that this is a new sub- species of chimpanzee - an exciting find, but not the same as finding the Loch Ness Monster.

Giant killer apes? Maybe. Large primates are extraordinarily powerful creatures and are capable of taking on animals such as leopards and lions.

Mostly, however, they sensibly choose not to. If these animals are big cat-hunters they would be unique. They seem to show no signs of aggression towards humans and definitely do not howl at the Moon.

But the evidence now is that in the forests of the Congo, there live large animals whose lives have been completely - until now - unknown to science. The question must be, what else is living in those green depths?

And, with peace and stability as far away as ever, how many hidden mysteries will succumb to the poacher's bullet before science gets a chance to confirm the myth?