Undersea Kingdom (Serial) (1936)

Deep below the sea is Atlantis, a lost world where two armies battle for supremacy. However the evil Unga Khan (Monte Blue) has pretty much sewn that war up. He now turns his attention to the world above (ours) and starts setting off earthquakes with one of his evil machines. In fact he has a lot of evil machines including death rays, tanks, flying warships and robot soldiers though for some reason most of his army still rides on horseback and fights with spears...

Professor Norton (C. Montague Shaw) owns a rocket machine and heads down below in it to see what is causing the earthquakes. He is accompanied by hero Ray (Crash Corrigan) and reporter Diana (Lois Wilde). They are soon trapped on Atlantis itself, can they stop the Unga Khan and save the world?

Amid many cliff hangers and ridiculous plots and traps they succeed of course. A highly entertaining, though frequently nonsense, serial though rather heavily based on Flash Gordon.