If you are not familiar with David Paulides' Missing 411 series, I highly recommend getting familiar with those books. Do NOT buy them on Amazon or Ebay or you will pay far too much - instead get them directly from Paulides at CanAm Missing Project.
A very short description is: Paulides became aware of unsolved / unexplained missing persons reports throughout the National Parks system - as well as in adjacent areas. Paulides has been criticized by some for presenting a few cases amongst the MANY he has found which possibly can be explained. These same folks take the fact that a few cases may be explained to mean that ALL of them can be explained. It's the same logic that applies to bigfoot research - one confirmed hoax is taken as evidence that it's all a hoax.
To get back on track - Paulides has noted several disturbing similarities amongst many of the cases, besides geographical locations. For instance, when the bodies are found, they are often found in places where Search and Rescue teams had previously looked. They are found often in boulder fields. Often, it is the last in line who gets taken. There are many, many other oddities woven into these strange cases - including toddlers found on the top of mountains which experienced climbers would have trouble ascending. Young children found 15 or 20 miles away from where they went missing, just a day or so later. The list goes on.
It isn't just children who go missing either - but people of all ages.
Paulides has identified "clusters" of missing people throughout the US/Canada (and even worldwide). While most of these are national parks and areas nearby, he has noted the entire state of Pennsylvania as a "cluster." Up until about the mid-20th century he noted that it was children who tended to go missing in PA. At some point, there was a shift, and now it seems to be men who are going missing in PA.
While Paulides has also been involved in bigfoot research and there are some hints in his book that bigfoot or something like it may be responsible for some of these disappearances - he never goes so far as to say exactly what or who he thinks may be responsible. Instead, he simply presents the evidence and lets the readers come to their own conclusions.
Here is a very interesting talk Paulides gave in 2013 in which he talks about bigfoot research, DNA, and his Missing 411 research:
Paulides and his son are set to release a Missing 411 documentary film this year.