In the tradition of such films as Plan 9 From Outer Space and Reefer Madness, this film starts with an introduction. In this case we get a lecture about hollow Earth theory. You can actually see the point where the lecturer becomes self conscious about his hands. From the multiple times he indicates the roundness of the Earth to the two seconds he puts his hands on his lapel he is a ballet of fidgety.
Then the movie starts which, it turns out, has absolutely nothing to do with a hollow Earth. Instead it's about a civilization that has grown over thousands of years underground in caves and tunnels.
The heroes of the movie are Beaver's dad and Shirley Temple's husband. Plenty of good stuff to riff (what kind of mushrooms are these people eating anyway?). Most of the underground denizens are extremely white (Johnny and Edger Winter would notice they were pale) with a few throwbacks that look like us. If you limit "us" to being fashion models (and beautiful ones, at that).
The ending is the best part. The two heroes escape with one of the throwbacks, and she gets to see the outside world for the first time. Of course, they come out high in the mountains so all she can see that's different from what she's used to is a bit of sky and some dead tufts of grass, but she finds it beautiful. John Agar explains to her that she hasn't seen anything yet. Then there's an earthquake and a pillar falls on her and kills her. I'm not sure what message the film makers were aiming for. Maybe a Buddhist message that life is suffering?