▲Paulides, among many others, uses the mysterious circumstances of these cold cases as potential evidence for the existence of these creatures.

▲Paulides reportedly submitted a FOIA request for records related to two missing hikers in Great Smoky Mountains National Park and noted on the request: "Associated with possible Bigfoot abduction."

▲The Wendigo is an evil spirit from Algonquin folklore, popular in Canada as well, who terrorizes forests and can possess humans, causing them to have cannibalistic urges.

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These theories have been criticized for many reasons, including their failure to let loved ones properly grieve.

Whether you believe the supernatural theories or not, the unsolved cases do hint at something very wrong.

▲Well, Paulides points to none other than Bigfoot, a hairy, ape-like giant that dwells in the wilderness and snatches up unsuspecting victims.

▲Paulides documents many of the cases in his book series ‘Missing 411,’ which he uses to highlight 52 clearly defined geographic "clusters."

▲A total of 84.9 million acres have been designated national park land, which makes up approximately 3.6% of all land in the US.

▲From Yosemite to Yellowstone, the US national parks are usually a source of pride and wonder.

▲Since the National Park Service was founded in 1916, over 1,000 people have vanished without a trace on the federal grounds.

▲The Rocky Mountains and Sierra Pacific range, as well as Colorado, Michigan, Georgia, and Arkansas.

▲The National Parks Service often leaves it to local law enforcement to track and solve the cases of missing persons, or to mark them as cold.

▲He found that several of the disappearances happened near berry patches and boulders, and that in many cases people were found at much higher altitudes than those at which they disappeared.

▲There are countless stories of people leaving for hikes and never returning, and even some where the bodies are actually found, but in areas that had already been extensively searched.

▲Stacey Ann Arras’s disappearance, untraceable even to a trained rescue dog's nose, along with hundreds of others, comprise a strange array of seemingly unsolvable national park vanishings.

▲After going a short distance ahead on a trail, he vanished. His body was found two weeks later, miles away, hundreds of feet higher in elevation, with all his snow gear and emergency equipment and provisions unused, and his shoes nowhere to be found.

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Take great care when traversing the stunning but dangerous land, and always let people know where you are!

See more: The weirdest things dubbed as "American" around the world

▲A toddler was found dead 12 miles from where he vanished, over two mountain ranges and numerous creeks, which he could’ve never traversed himself.

▲Despite the controversy, Paulides’ research did discover something scary: the fact that the National Park Service doesn't keep track of missing persons, and can’t even name exactly how many people have disappeared.

▲Particularly in cases where people appeared to vanish into thin air, he noticed that the rescue dogs either could not or would not perform their duties.

▲With a federal database, hopefully commonalities will be easier to pinpoint, and the rate of disappearances will decrease.

▲In one case, a physician, in excellent physical condition and equipped with new snow gear, went hiking to a winter cabin with his friends. What could go wrong?

▲With the revelation that there is no federal-level database tracking how many people go missing from federal lands, Paulides’ work kick-started a petition to hold the National Park Service responsible.

▲Paulides showed particular interest in the cold cases involving the National Parks Service, and began compiling any data he could collect to look for trends.

▲Tragic accidents are inherent risks of exploring the wilderness. Drowning is the most common accident, but there’s also dangers like falling from the peak of a mountain, animal attacks, and extreme weather.

▲The cryptozoologist and former police detective introduced a new theory that has quickly taken hold of many.

▲Something wicked lurks beneath this majestic wilderness.

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After interviewing families, law enforcement, and search-and-rescue personnel, poring over newspaper archives, and submitting hundreds of Freedom of Information Act (FOIA) requests, all signs pointed to Bigfoot.

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The internet is obsessed with the unsolved mystery of the girl who, on a horseback riding trip in Yosemite, went off to take a photo by the water then vanished, leaving nothing but her camera lens behind.

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Over 1,000 people have vanished from American national parks in the past 100-odd years, often leaving no trace behind. Check out this gallery to learn more about the freakish cases and the shocking reason behind them.