[[caption-width-right:350:[[WesternAnimation/SpongebobSquarepants Down, down, down, to the bottom of the sea...]]]]
-> ''Nossing in ze vorld can ztop me now!''
-->-- What many fans think '''Zaroff''' said.

-> ''Nothing in the world can stop me now!''
-->-- What '''Zaroff''' really said.

The one where Jamie slaps Polly.

The Doctor, with Polly, Ben and Jamie in tow, lands the TARDIS on a volcanic island where they are captured and taken into a cave complex. It soon becomes clear that this tiny island is all that remains of the ancient Atlantean civilisation.

Mad scientist Professor Zaroff (played with a LargeHam by Joseph Furst) has convinced the Atlanteans that he can raise the city from the sea, but in reality he plans to drain the ocean into the Earth's core and thereby destroy the planet. [[ForTheEvulz Just because]], OK?

Polly is taken off to be converted into an aquatic FishPerson, while the others meet two shipwrecked sailors who help them convince the Fish People to rebel. Eventually, the Doctor manages to foil Zaroff's plan by flooding the city before he can complete the drilling. Zaroff drowns, but everyone else makes it out and the Doctor and companions depart.

For a long time it was thought that only episode 3 remained, until [[http://www.scifinow.co.uk/news/doctor-who-galaxy-4-and-the-underwater-menace-missing-episodes-found/ episode 2]] turned up in 2011.

* ActuallyPrettyFunny: Zaroff is only very briefly angry when he learns that the Doctor tricked him into saving his life with a [[BlatantLies blatant lie]].
* AllThereInTheManual: In an early draft of the story, and in the novelisation, Zaroff's motivation to blow up the Earth stemmed from his wife and children dying in a car crash and driving him mad.
** The novelisation adds many missing details:
*** It gives Zaroff's first name as Hermann.
*** Zaroff has been missing for twenty years.
*** Zaroff created the fish people through DNA resequencing based on Atlantean legends.
*** It is stated by Ara that Zaroff walks through the market place every day to inspect the drill head, thus this is where the Doctor stages his kidnap attempt. In the television story, the only reason he goes through the market is to look for the Doctor.
*** Ara is said to be the daughter of a councillor who was killed after speaking out against Zaroff, with Nola having worked for the family.
*** It is explained by Sean that the Atlanteans do not settle on the surface because they consider their ground sacred.
*** Ara says that the grille behind Amdo's statue was set up by the original priests to control the population, and her father showed it to her before he died. It is also said that even Lolem does not know about it.
*** It is said that Zaroff saved Thous from a deadly disease thought fatal by the Atlanteans, thus why Thous trusts Zaroff more.
* {{Atlantis}}: Or at least the last remnants of that civilisation, living underground.
* AuthorFilibuster: Against organised religion in part 4.
* BigBad: Professor Zaroff.
* BodyHorror: Polly almost has surgery to turn her into a fish-person.
* BrokenAesop: As noted, the AuthorFilibuster has the Atlanteans blame [[ReligionIsWrong their religion for all their problems.]] Even though the actual culprit Zaroff didn't seem to have much of anything to do with their religion. Or that one of the few Atlanteans to see through Zaroff's deceptions and distrust him (even before the Doctor's help) was one of the priests of their religion. And the character making the Filibuster had been an antagonist who hadn't done anything good, sympathetic, or redemptive up to this point. The Filibuster was apparently supposed to be that character's redemption.
** Also, the prophecy did come true in a way.
* ChewingTheScenery: Zaroff was a fandom injoke for how much he milks the line "NOTHINK IN ZE VORLD CAN SHTOP ME NOW!!", which is used as TheStinger at the end of the [[https://www.youtube.com/watch?v=Pp4x22u6SBU official DVD trailer]]. And, of course, "BANG! BANG BANG!"
* {{Chickification}}: This story is particularly extreme in depicting Polly as a DistressedDamsel with no redeeming moments of agency, to the displeasure of the actor Anneke Wills.
* {{Cyborg}}: The fish-people, humans fitted with 'plastic gills' and artificial eyes that allow them to see better underwater.
* DareToBeBadass: Sean uses this approach to persuade the Fish People to go on strike.
* DressedAllInRubber: Zaroff's thugs (and while impersonating them Ben and Jamie) wear rubber wetsuits.
* EarthShatteringKaboom: What Zaroff hopes to create.
* EvilIsHammy: Zaroff could have a HamToHamCombat with the Daleks and have a good chance of winning.
* FishPeople: Created by surgery rather than a natural species. Polly almost becomes one in some of the few clips to survive from episode 1 and in the newly-recovered episode 2.
* [[ForScience For SCIENCE!]]: The closest Zaroff comes to an explanation of his motives in the TV story (but see the All There In The Manual entry above) is pretty much this.
--> '''The Doctor''': Just one small question. Why do you want to blow up the world?
--> '''Zaroff''': Why? You, a scientist, ask me why? The achievement, my dear Doctor. The destruction of the world. The scientist's dream of supreme power!
* ForTheEvulz: Zaroff tries to destroy Earth just for the fun of it.
* GodGuise: Ben prevents the execution of the Doctor and Ramo by speaking from inside the hollow idol of Ando and pretending to be the voice of the goddess.
* HumanSacrifice: The Atlanteans try to sacrifice the Doctor and company to the goddess Ando... using sharks.
* ICantBelieveImSayingThis:
-->'''Jamie [=McCrimmon=]:''' You know I never thought I'd say this, but well it's great.
* LaResistance
* MadScientist: Zaroff is a classic example.
** At the end of the Episode 2 clip on the BBC site, he sounds like a proto-Davros, on how succeeding with his [[OmnicidalManiac omnicidal mania]] would be the ultimate power.
* MadeASlave: The Atlanteans enslave all outsiders who stumble on their land, except for Zaroff.
* {{Mythpunk}}
* ObscuredSpecialEffects: The KnockoutGas used by the Atlanteans is invisible although this overlaps with FridgeBrilliance, because the Doctor explains it's high-pressure nitrogen, presumably to invoke the famous diving hazard of "the bends" where nitrogen under high pressure begins to act on the body like laughing gas (of course, in reality you'd probably just suffocate, but it goes with the undersea theme of the episode quite well).
* OminousLatinChanting: multiple times in the temple of Amdo.
* OutOfCharacterMoment: There's a scene where Polly is trapped in a flooding chamber and [[HystericalWoman panicking and screaming that she can't do it]], and Jamie pulls her to her feet and [[GetAHoldOfYourselfMan slaps her on the face]]. This was in a MissingEpisode, and when Creator/TheBBC's narrated audio reconstruction was made, Anneke Wills and the director both agreed that Jamie [[CanonDiscontinuity didn't slap Polly]] because it didn't fit his character or their relationship, and they felt that Jamie dragging Polly to her feet would have been enough to snap her out of it.
* ReligionIsWrong: The conclusion the surviving Atlanteans reach through [[BrokenAesop dubious]] [[AuthorFilibuster means]] at the end.
* StrappedToAnOperatingTable: Polly, to be turned into a Fish Person. The scene is disturbing enough to have been cut in Australia and to have provoked protests from MoralGuardians that it would make children frightened of doctors.
* SuicidalCosmicTemperTantrum: Zaroff.
* WhatTheHellIsThatAccent: [[RealityIsUnrealistic It's Joseph Furst's real one, that's what]].