Athlete, navy-man, and overall übermensch 'Crash Corrigan' (stuntman Ray Corrigan, who subsequently went by Ray 'Crash' Corrigan in films and TV work), along with intrepid 'girl-reporter' Diana (Lois Wilde), young sidekick Billy (Lee Van Atta), avuncular boffin Professor Norton (C. Montague Shaw), and comic-relief buddies Briny and Salty (Smiley Burnette and Frankie Marvin) travel by rocket-propelled submarine to the lost continent of Atlanta, where they are immediately caught up in a war between the evil despot Unga Khan (Monte Blue, stealing cues from Flash Gordon's faux-Asian arch-enemy Ming the Merciless) and his black-robed henchmen (including Lon Chaney Jr.) and good-guy Sharad (William Farnam) and his white-robed followers (for a 'nice guy' Sharad seems ruthless enough when presiding over blood-sports in the Atlantean coliseum). Everyone's near reverence for Crash gets a bit smarmy after a while (or in Billy's case a bit creepy), especially considering the hero's outfits occasionally border on campy-outrageousness (skimpy fish-scale briefs and a head-piece sporting a prominent fin) but the stuntman and gorilla-imitator turned actor acquits himself pretty well as a cunning, quick-fisted, and resourceful hero (the rest of the cast are functional and nondescript). The Republic serial is very similar to Mascot's popular 'Phantom Empire' 12-parter (1935) starring Gene Autry: both feature a technologically advanced 'lost' kingdom in political ferment, incredible 'scientific gadgetry' including ray-guns, mechanical men, 'flying torpedoes', and viewing machines (that can see anywhere, no camera needed), and both chapterplays conclude apocalyptically which oddly don't seem to bother the heroes, who are now safe on the Earth's surface. As depression-era serials go, 'Underwater Kingdom' is typical, and 'OK'. The storyline is simplistic, there are numerous unacceptable implausibilities (Billy can pilot one of the Unga Khan's flying machines!?), characters seem to inexplicably know the names of things (such as 'vol-planes' 'volkite' robots) when first encountered, and several of the cliff-hanger resolutions are 'cheats' (the 'before' and the 'after' footage don't match). Not surprisingly, production frugality is evident, notably in frequently repeated scenes (when the submarine ascends, bubbles are sucked back into the engines) but some investment and imagination went into the fearsome 'Juggernaut' (I like the sound it made) and the lumbering volkites (goofy but much less ridiculous than the behatted tinmen menacing Gene Autry in the Phantom Empire. More interesting as history than as entertainment but fans of vintage sic-fi should enjoy it in a smug, eye-rolling way (as I did).
