Try also the 1950s. I'm not so sure the article was about caves or Missouri, but I cannot offer better. I do recall the photograph in "National Geographic" showing a skull with the Civil War forage cap beside it. The caption was wondering if the soldier came into the cave for some reason and then simply died there. I'm sorry I do not recall more. The article itself gave no more detail on the mystery--only the photo and its caption, if memory serves.
I've always wondered about that one, too.