Undersea Kingdom (1936) is a Republic Pictures 12 chapter film serial released in response to Universal's Flash Gordon.

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.

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.

[3] Along with him in his Rocket Submarine are Crash, Diana, three sailors (Briny Deep, Salty, Joe) and their pet parrot Sinbad.

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.

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.

These include the Volkites (The robotic army of the Black Robes), The Juggernaut (their tank), Vol Planes (Atlantean rocketships) and Professor Norton's Rocket Submarine, as well as sets and Unga Khan's tower.

No cameras or microphones are apparently necessary to this second function and it appears to have no limits - viewing the Sacred City, areas of Atlantis and the upper world with no problems.

In the last chapter, an Invisible Wall of Atom Rays is activated to provide Unga Khan's tower with a force field to protect it from the United States Navy's bombardment.

The most advanced technology seemingly available to the opposing White Robes of Sharad are catapults and flame throwers built into the walls of the Sacred City.

[1] In the words of film historian William C. Cline, Undersea Kingdom was a "totally unbelievable – but visually enjoyable – twelve-chapter madhouse chase.