The search for Scotland's Loch Ness Monster is world famous. Far less well-known is the hunt for a similar creature, Mokele-mbembe, which is reputed to live in the remote north of Congo-Brazzaville. But how strong is the evidence?
"I checked maps, and the data on the maps was white. It said, 'insufficient data to delineate terrain'. Well that got me!" says Dr Roy Mackal, a retired biologist from the University of Chicago.
"It's the end of the world. It gives you a feeling of a surviving prehistoric time."
In the 1980s, Dr Mackal led two expedition teams to the vast Likouala swamp and rainforest area of the Congo which is inhabited by pygmies, on the hunt for this mystery creature - Africa's version of Scotland's Loch Ness Monster.
The Mokele-mbembe is reputed to be a large reptile-like creature, with a long neck, and long tail.
Despite being a herbivore, it is said to roar aggressively if approached by humans. Some say it has a single horn, which it uses to kill elephants.
Many a Western explorer over the years has been gripped by the tantalising possibility that they could discover a creature - a formidable one at that - that has remained, as yet, unknown to science.
Rising 'out the water'
To date, there have been more than 50 expeditions to the region, but no scientific evidence, unless you include the large claw-shaped footprint recorded by a French missionary in 1776, and by a number of others since.
The only photographic images have been so fuzzy, they prove nothing.
But there is no shortage of eyewitness reports.
"I was in a boat on the river when I saw Mokele-mbembe. He began to chase us. Mokele-mbembe rose out of the water," one man told the BBC. "We ran, or he would have killed us."
Paul Ohlin, a community development worker who spent more than 10 years living with the Bayaka in Congo and the Central African Republic, just to the north, says the people who live in the area are in no doubt about the creature's existence.
"When people are sitting around the campfire talking, they talk about the Mokele-mbembe - it's something that's a reality in everyday life," he says.
At the same time he emphasises their "spiritual connection" and "mystical relationship" with it.
"The way they see the world is a little different to the way you and I see it," says Paul.
But their eyewitness reports still need to be taken seriously, in his view.
"Certainly mythology surrounds it," says Adam Davies, a British man who spends his spare time and money travelling the world in search of undocumented species, and has twice gone to Africa on the trail of the Mokele-mbembe.
"But when you put it to people, 'Is this a real creature?' they become quite affronted… and they consistently came out with physical descriptions."
"Never dismiss tribal accounts on the basis that they must be talking tosh because they are tribal - that's not right and it's actually disrespectful," he says.
The field of cryptozoology - the search for large, unproven species - extends well beyond the realms of mainstream science.
But those who believe Mokele-mbembe exists point out that some animals once dismissed by science have turned out to be real.
The most often cited example is the okapi - a cloven-hoofed mammal with zebra-like stripes on its legs, which lives in the Democratic Republic of the Congo, just to the east of Congo-Brazzaville.
In the 19th Century, there was talk among Westerners in Africa of the existence of an "African unicorn" and the explorer Henry Morton Stanley - who had earlier tracked down the missing missionary, Dr David Livingstone - reported seeing a mysterious donkey-like animal on a journey through the Congo in the late 1880s.
It was only in 1901 that the okapi was properly documented and identified as a relative of the giraffe.
"I'd put Mokele-mbembe in the same category as the Loch Ness Monster," says Bill Laurance, professor at James Cook University in Australia, a conservation biologist and an expert in tropical rainforests.
"My gut sense is that the likelihood of the creature actually existing today is small.
"However, one thing you learn early on in science is never say never. We are still discovering new species all the time."
The Likouala region in the north-east of Congo Brazzaville is the kind of place that it is easy to imagine containing hidden mysteries. Congolese government officials say 80% of its 66,000 sq km is uncharted. Much of it is dense, often flooded forest, forming part of the second largest rainforest in the world.
"The idea of a creature which is very rare, living in a very remote area with a vast size to it, is not remotely implausible," argues Adam Davies.
But some wonder about the motivations of the Congolese who promote the existence of the creature.
US writer Rory Nugent who went to Congo in search of the Mokele-mbembe and wrote a book about his experience, Drums Along the Congo, says he saw "an elegant French curve moving through the water".
He believes it might have been the head of the famed creature, but he is also deeply sceptical.
"The guides were screaming about a god beast. Whether it was part of the show, whether there was somebody swimming under the water with flippers pushing a cardboard piece across the lake, I couldn't tell you."
Taking foreigners on expeditions to try to find the Mokele-mbembe is a good "money making operation" for those involved, he adds.
Mr Nugent fears that one day a kind of "Disneyland Congo" could be created in the area - similar to the tourist trap around Loch Ness - with scientists and tourists from the world flying in and out.
Those who believe the Mokele-mbembe exists argue that with further dedication of time and resources, one will eventually be tracked down.
But might the discovery of the creature be an anti-climax? Perhaps the mystery is what we enjoy most.
"I think there is a basic need or drive to entertain possibilities just outside of our reach," says psychology professor Jacqueline Woolley of the University of Texas.
"There is the excitement in believing that what seems impossible or improbable could potentially exist."
She says that for belief in creatures like the Mokele-mbembe to take hold, they "can't be too wacky and far out - they must be similar to real entities," but vary in just one or two ways.
"I realise my bias," admits Dr Mackal, who is now in his 80s. "I'm interested in discovering unknown species of animals."
"But I think that Mokele-mbembe still exist, and there isn't just one - they are reproducing," he contends.
"At 86 years old, I would dearly love to be alive if and when the animals are discovered."