Undersea Kingdom (1936) is a Republic Pictures 12 chapter film serial released in response to Universal's Flash Gordon. It was the second of the sixty-six serials made by Republic. In 1966, the serial was 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. Thus, he will rule the world unless he can be stopped in time.

The star of the serial is Ray "Crash" Corrigan, using that screen name for the first time. The name was created to sound similar to "Flash Gordon", in one of many similarities. Formerly a stunt man – he was the person swinging on vines in Tarzan the Ape Man (1932) – Corrigan went on to use this screen name for the rest of his career in serials and B-Westerns.

The first two chapters of the serial were mocked on the TV show Mystery Science Theater 3000 in July/August 1992.