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. Thus he will rule the world unless he can be stopped in time.

Plot
The plot involves the main character “Crash” Corrigan trying to stop an evil tyrant ruler of Atlantis from conquering the lost continent and then the entire upper world.

Lieutenant Crash Corrigan, in his last year at Annapolis, is invited by Billy Norton to visit his father, Professor Norton, after a wrestling match. At their house, the professor is demonstrating his new invention, which can detect and prevent (at short range) earthquakes, to Diana Compton and his theory about regular tremours from the area where Atlantis used to be.

When Atlantean tyrant Unga Khan and his Black Robe army turn their Disintegrator beam on St Clair, Professor Norton leads an expedition to investigate. Along with him in his Rocket Submarine are Crash, Diana, three sailors (Briny Deep, Salty, Joe) and their pet parrot Sinbad. Unknown to the expedition until it is underway and in trouble, Billy has stowed away on the Rocket Sub as well.

Problems for the expedition begin when Joe, in charge of the engine room, is driven mad by the fear that the submarine cannot survive such depths. In order to prove this, he locks the engine room door and sends the sub into a fatal dive. As soon as this crisis is averted, Unga Khan and Captain Hakur detect their approach and bring them through a tunnel into the Inland Sea with a Magnetic Ray.

Cast
Ray “Crash” Corrigan as Crash Corrigan, Athlete and Navy Lieutenant. Crash Corrigan’s stage name came from this serial.
Lois Wilde as Diana Compton, Reporter from the Times who joins the expedition to Atlantis to get a story
Monte Blue as Unga Khan, Tyrant and leader of the Black Robes
William Farnum as Sharad, High priest of Atlantis and leader of the White Robes
Boothe Howard as Ditmar, one of Unga Khan’s Black Robes
Raymond Hatton as Gasspom, one of Unga Khan’s Black Robes
C. Montague Shaw as Professor Norton, a British-born US scientist and leader of the expedition to Atlantis in his Rocket Submarine
Lee Van Atta as Billy Norton, Professor Norton’s young son who stows away aboard the submarine expedition
Smiley Burnette as Briny Deep, a sailor in Professor Norton’s Rocket Sub expedition and comic relief
Frankie Marvin as Salty, a sailor in Professor Norton’s Rocket Sub expedition and comic relief
Lon Chaney, Jr. as Captain Hakur, Black Robe commander, Unga Khan’s main henchman. Captain Hakur was “probably” Lon Chaney, Jr.’s “most colourful henchman.”
Lane Chandler as Darius, one of Sharad’s White Robes
Jack Mulhall as Lieutenant Andrews, US Navy
John Bradford as Joe, a sailor in Professor Norton’s Rocket Sub expedition who goes mad with fear and almost kills them all
Malcolm McGregor as Zogg, one of Unga Khan’s Black Robes
Ralph Holmes as Martos, one of Sharad’s White Robes
John Merton as Moloch, Black Robe whose life is spared by Corrigan and switches sides
Ernie Smith as Gourk, one of Unga Khan’s Black Robes
Lloyd Whitlock as Captain Clinton, US Navy Captain