Dan Campbell was illegally hunting elk antler sheds in Yellowstone Park when he disappeared more than three decades ago.

Campbell had set off from Hellroaring Trailhead to Jardine, Montana, outside of the park's boundaries, with his dog Freckles for a four-day trek.

However, when the 42-year-old Montana man failed to appear at the designated location and time, his girlfriend, Tracy Erb, reported him missing April 8, 1991.

Campbell is one of at least six people to mysteriously disappear in the park since the late 1960s.

There may be more, but it’s difficult to get a handle on the numbers.

Officially, the park doesn’t appear to track the missing. When asked for a list of missing people, the Yellowstone Park Public Affairs Office referred Cowboy State Daily to the national park’s cold case database containing 25 missing persons cold cases for all national parks.