''Atragon'' (aka ''Kaitei Gunkan'', or ''Undersea Warship'') is a 1963 tokusatsu film from the producers and crew of ''Franchise/{{Godzilla}}''.

The lost kingdom of Mu, which sank beneath the sea over twelve thousand years ago, launches an assault to make the upper world their domain again. With an array of weaponry, the Mu Kingdom wages a war with the surface dwellers, who counter with the super-scientific battleship Atragon.

As the Atragon attempts to defeat the Mu Kingdom in their own underwater base, the Mu unleash the legendary sea dragon Manda against the humans.

This movie introduces Manda to the Creator/{{Toho}} universe, who was later incorporated into the Godzilla universe. It also marks the first appearance of the Gotengo[=/=]Atragon, which became one of Toho's signature military battle vehicles. The Atragon appears 2004's ''Film/GodzillaFinalWars'', which recreates the battle between the Atragon and Manda from this film.

See also ''Anime/SuperAtragon''.
!!Tropes that apply to ''Atragon'' include:

* CallingTheOldManOut: Captain Jinguji is so fixated in restoring Japan's honor after the nation's defeat in World War II that everyone, including his own daughter, calls him out on this. Granted that Nationalism and Patriotism is the basic themes of this film, but he actually took it too far.
* CoolShip:
** The Gotengo/Atragon.
** Subverted with the Red Satan, supposed to be a super-advanced submarine but is destroyed after one encounter when it pursues a Mu submarine past crush depth.
* DoomsdayDevice: Mu's seismic generator.
* DidYouJustPunchOutCthulhu: The Atragon froze Manda to death.
* DubNameChange: Gotengo to Atragon.
** The film itself was renamed by Toho for greater appeal in overseas distribution, Atragon being a portmanteau of the English words ''Atlantis Dragon'', a clear reference to Manda. The net result of this ExecutiveMeddling is that the movie's native-language title refers to the protagonist supership, and its overseas title does the same, but said overseas title was actually created in reference to the kaiju antagonist... a dragon that technically isn't even from Atlantis!
* TheDragon: Manda literally is one, but he's a guardian monster
* GenreBusting: Toho's war films were very popular during the 1960s, so writer Sekizawa came up with the idea of combining Toho's popular war films with their fantasy/kaiju films and voila...
* IntrepidReporter: Played with. Umino seems to take his job seriously in an odd matter. Later defied when he was [[TheMole a Mu Agent all along]].
* NoNameGiven: The Mu Agents and the Empress. As well as her Chancellor.
* PapaWolf: It took Captain Jinuji a long ass time to see he should use the Atragon to save his daughter. And he ''does''.
* RentAZilla: Manda
* SceneryPorn: The panoramic shots of the kingdom of Mu are breathtaking. Tsuburaya-sama outdid himself on this film.
* TheStoic: Captain Jinguji, played by the master of stoics, Jun Tazaki.
* YamatoNadeshiko: The Empress of Mu.