Undersea Kingdom (1936) is a Republic Pictures film serial released in response to Universal's Flash Gordon. It was the second of the sixty-six serials made by Republic. In 1966, scenes from the serial were edited into a 100-minute television film titled 'Sharad of Atlantis'.

Following a suspicious earthquake, and detecting a series of signals, Professor Norton leads an expedition, including Lt Crash Corrigan and Reporter Diana Compton, in his Rocket Submarine to the suspected location of Atlantis. Finding the lost continent, they become embroiled in an Atlantean civil war between Sharad (with his White Robes) and the usurper Unga Khan (with his Black Robes) who wishes to conquer Atlantis and then destroy the upper world with earthquakes generated by his Disintegrator. He will rule the world unless he can be stopped in time.