A forensic expert in the US believes he has some of the strongest evidence yet that the Bigfoot, or sasquatch, creature exists.
Much bigfoot evidence turns out to be fake
The creatures are real enough to those who say they have spotted them - but most scientists remain sceptical about their existence.
Investigator Jimmy Chilcutt of the Conroe Police Department in Texas, who specialises in finger and footprints, has said he believes he is certain around six footprints found - claimed to have been made by Bigfoot - are genuine.
He added that one 42 cm (18-inch) print found in Washington in 1987 has convinced him.
"The unique thing about this cast is that it has dermal ridges - and the flow and texture matches the ridge flow texture of one from California," Mr Chilcutt told BBC World Service's Discovery programme.
"The ridges are about twice as thick as in a human being."
Before becoming involved in bigfoot studies, Mr Chilcutt had amassed a huge collection of ape and monkey prints as part of a police research project.
He added that the ridge flow pattern was crucial in proving the prints had not been made by a very large-footed human or other primate.
"The ridges run down the side of the foot - in humans, the ridges run across the width of the foot," he said.
"That's what makes it unique. The only other animal I've seen this in is a howler monkey in Costa Rica.
"As a crime scene investigator, I don't deal in what I believe or what I think.
"I examine physical evidence and make a determination... I know there's an animal out there, because I've seen the physical evidence."
The Bigfoot is considered to be a North American version of the yeti of the Himalayas. The name bigfoot comes from several huge, mysterious foot impressions found in 1959 in a Californian forest.
Hundreds of other prints have been found since, although many have turned out to be hoaxes.
"There have been reported sightings in every state of the United States, other than Hawaii and Rhode Island," said Craig Woolheater, director of the Texas Bigfoot Research Center.
"It's not the missing link, it's not an extra-terrestrial, it's just an animal - a flesh-and-blood primate that has learned to be elusive around man and avoids man where possible."
Mr Woolheater's organisation investigates about 100 Bigfoot sightings in the state each year - as well as the surrounding states of Arkansas and Louisiana.
Members use a wide range of technology - remote-controlled cameras, video surveillance systems, night-vision, and thermal imaging - in an effort to get video and photographic evidence of these creatures.
Sceptics believe sightings are tricks of the mind
So far it has proved unsuccessful.
However, other evidence gathered through time includes footprints, audio recordings and "limb twists" - where branches of trees have seemingly been twisted by a type of primate with massive strength.
These twists are a common aspect of primate behaviour and Bigfoot hunters say they occur in areas where there have been a number of sightings.
But most of the evidence - such as photographs, hair samples, and even blood - has turned out to be fake.
"There is a significant amount of evidence for Bigfoot - there are tracks, there are fuzzy photographs, there are hair samples, there are sighting reports - the problem is that it's not good evidence," said Benjamin Radford, managing editor of Sceptical Inquirer magazine.
"I liken it to a cup of coffee - if you have many cups of weak coffee, they can't be combined into strong coffee.
"It's the same with scientific evidence. If you have lots of weak evidence, the cumulative effect of the evidence doesn't make it strong evidence - and what science needs to validate a Bigfoot is strong evidence."
Bigfoot is probably the best-known of the subjects of "cryptozoology" - the study of hidden creatures.
Some scientists are highly sceptical, believing these creatures to be nothing more than tricks of the mind.
"One of the problems - and I know this from my background in psychology - is that it's actually fairly easy to fool ourselves," said Mr Radford.
The Bigfoot prints are similar to howler monkeys
"What often happens is that people will be out in the wilderness and they'll see something out of the corner of their eye - something dark or hairy or fast - that will surprise or shock them.
"If they're already thinking that there's a Bigfoot in the area, it's easy to make the leap between saying: 'I saw something, I don't know what it is,' to: 'I saw something and it's Bigfoot.'"
But others say it is best to keep an open mind.
"Every now and again big things turn up," Colin Tudge, zoologist and author of the book The Variety Of Life, told Discovery.
"The okapi - a horse-sized relative of the giraffe - turned up only in the early 20th Century.
"A few years ago somebody discovered an absolutely enormous shark in the ocean.
"The most recent - and I think the most spectacular - is an animal that people think is a goat-antelope, some kind of relative of the shamuar, which has turned up in the forests of Vietnam.
"This is an animal about the size of a Shetland pony with long horns, that nobody even suspected was there until just a few years ago - it was finally identified in about 1994."