Help! Trying to identify an old animated film.

I've been trying to remember the name of an animated film I saw on tv in the late fifties or early sixties. It was 2-D animation (no puppets). The plot involved a little boy who had an adventure underground, with maybe some goblins or something similar. He was trying to rescue someone (or something), I think. Not much help, I know.

I saw it when I was a kid in the St. Louis area. They used to have a local kids show called "Cookie and the Captain." They were supposedly on the submarine "S.S. Popeye" and showed all the old Popeye cartoons and Little Rascals shorts, but also a lot of old serials (Commander Cody, Jungle Jim, etc.)

I can't remember if the animated film I'm describing was a serial or just a feature that they manually chopped up for their format. It might have been foreign, but I don't really remember. It was fairly scary, and it's been driving me nuts for decades...