The Phantom Empire

The Phantom Empire is a 1935 American Western serial film directed by Otto Brower and B. Reeves Eason and starring Gene Autry, Frankie Darro, and Betsy King Ross. This 12chapter Mascot Pictures serial combined the western, musical, and science fiction genres. The first episode isminutes, the rest aboutminutes. The serial film is about a singing cowboy who stumbles upon an ancient subterranean civilization living beneath his own ranch that becomes corrupted by unscrupulous greedy speculators from the surface. In 1940, a 70minute feature film edited from the serial was released under the titles Radio Ranch or Men with Steel Faces. This was Gene Autrys first starring role, playing himself as a singing cowboy.

A chance to become real heroes occurs when Betsy, Frankie, and Gene are kidnapped by the real Thunder Riders from the superscientific underground empire of Murania, complete with towering buildings, robots, rayguns, elevator tubes that extend miles from the surface, and the icy, blonde, evil Queen Tika. On the surface, criminals led by Professor Beetson plan to invade Murania and seize its radium wealth, while in Murania, a group of revolutionaries plots to overthrow Queen Tika.The inhabitants of Murania are the lost tribe of Mu, who went underground in the last glacial period 100,000 years ago, and now live in a fantastically advanced city 25,000 feet below the surface. They cannot now breathe the air at ground level and must wear oxygen masks. Surface dwellers have no trouble breathing Muranian air. The Thunder Guard emerges to the surface world from a cave with a huge rock door that swings up like a garage door. Both Muranians and Professor Beetson want to get rid of Autry, so he loses his radio contract and Radio Ranch is vacated. ........

Source: Wikipedia