This episode, and the previous one, The Unknown People: Part I (#1.25), was originally part of theatrical film Superman and the Mole-Men (1951). The producers of the series later edited the film into episodes of the show.
This is the last episode to have Jack Larson, John Hamilton, and Robert Shayne not listed on the credits. Starting with the next episode, all three cast members will be listed every episode onward, whether they appear or not.
This episode is the second and final part of the only two-part (or multi-part) episode in the entire series.