In the 50s and 60s, there were over hundred films were not flashy enough (or bad enough) to get the main-stream's attention. Most were too average to leave a lasting impression. Quite a few, however, were intriguing and entertaining films which don't deserve their obscurity. They may not be as readily available commercially -- a bit off the beaten path -- but are examples of good non-mainstream sci-fi. (which relied on story more than special effects)
Below is my list of ten fun "buried treasure" sci-fi films. There are more than ten, of course, so readers should feel free to post their own favorites via the Comments feature.
1. Red Planet Mars ('52) -- A scientist invents a special radio which makes contact with a civilization on Mars (or is it God?). A duplicitous former nazi employed by the Soviet Union is out to discredit or destroy the scientist.
2. It Came From Outer Space ('53) -- Strange beings from outer space crash-land near a desert town. Residents begin to disappear, then reappear, but acting strangely. Is it an invasion, or are the aliens up to something else?
3. Not of this earth ('57) -- An alien on earth needs regular blood transfusions to survive. He can kill with a look of his eyes. He is the vanguard of a project to harvest earthlings to help his race on their home planet survive. A doctor begins to unravel the plot.
4. Kronos ('57) -- An alien civilization which consumes far more power than they can create, send a giant power-absorbing robot to earth to steal earth's electrical energy. The robot is impervious to earth's weapons, simply absorbing their power too.
5. X: The Unknown ('57) -- A radioactive blob arises from a crack in the earth. It seeks out medical or research isotopes as food. It kills anyone in its path. Is it alive? How can it be stopped?
6. Incredible Shrinking Man ('57) -- A man is exposed to a radioactive fog. He begins to shrink. A personal tragedy becomes a fight for survival when he is a small as a spider's lunch.
7. Colossus of New York ('58) -- A distraught scientist creates a mechanical body for the brain of his adult son (killed in a traffic accident). Intending that his son finish his work to solve world hunger, the brain-machine hybrid causes the son to slowly become unhinged.
8. The Mysterians ('59) -- Aliens land on earth looking for just a few square miles of land to live on, and a few earth women to rebuild their species. Was it a sincere request for help, or a smoke screen for a darker intent?
9. The 4D Man ('59) -- A scientist subjects himself to a special radioactive process. He is then able to alter his body's "time", allowing him to move through solid matter. His new power also kills by draining the life out of people. The power begins to affect his mind.
10. Creation of the Humanoids ('62) -- In earth's future, human population is in decline. A growing population of servant robots eventually outnumber the humans. Anti-robot vigilante squads discover a robot plot. Is it to seize power or something else?
For you 50s/60s sci-fi fans out there, what "buried treasure" movies would you nominate?