Undersea Kingdom (1936)

Starring Ray Corrigan, Lois Wilde, Monte Blue, William Farnum, Boothe Howard
Directed by B. Reeves Eason & Joseph Kane
(actor & director credits courtesy IMDB.com)

Navy lieutenant "Crash" Corrigan and his friends trace a mysterious force causing earthquakes to the lost city of Atlantis, where a powerful despot is making plans to conquer the world.

Here's an action-packed and fun movie serial from the early years of the Republic Pictures studio, which per Wikipedia was formed from a number of smaller studios including Mascot Pictures, who provided much of the principal crew for Republic's serial unit.  Mascot's serial a year earlier, The Phantom Empire, seems to have inspired this production, which also features a lost civilization, lumbering robots, and plenty of men on horseback.  Although no one would call this a great science fiction opus, it's nonetheless very enjoyable, enlivened by thrilling music, chariot chases, sieges of a fortress, sword fights, and a great deal of derring-do for Corrigan.  Although no one particularly distinguishes themselves in the cast, Lon Chaney, Jr. some years before becoming a horror star, plays the captain of Blue's soldiers.