The Phantom Empire (1935)

When the ancient continent of Mu sank beneath the ocean, some of its inhabitant survived in caverns beneath the sea. Cowboy singer Gene Autry stumbles upon the civilization, now buried ... See full summary »

Directors:

 Otto Brower, B. Reeves Eason (as Breezy Easton)

Writers:

 Wallace MacDonald (story), Gerald Geraghty(story), 4 more credits »

Stars:

 Gene Autry, Frankie Darro, Betsy King Ross - via IMDB 

https://archive.org/details/The_Phantom_Empire_1935