Cataclysm Climax

An Ending Trope common to B-Movies of the 50s and 60s, especially those involving a Lost World, in which an earthquake, volcano, avalanche or some similar (usually natural) disaster destroys the Lost World. This can be hinted at in advance, but as often as not it appears out of the blue to threaten the main characters and necessitate a hasty departure. Also note that the disaster always manages to hold off just long enough for the characters to actually discover the Lost World and explore for a bit, then literally yanks the ground out from under them.

Of course, in a few cases the cataclysm is actually not a coincidence, but indeed is triggered by the protagonists. In these cases it's usually unintentional; it could be the heroes' very presence which somehow disrupts the natural balance. Or, sometimes it is set off by some act done by the villains, or occasionally the heroes — say, a stray bullet/laser blast/etc meant for the heroes instead hits a support structure or the ancient codex holding the world together or what have you. Crash!

If the Lost World in question is an island, an underground society or otherwise isolated location, expect it to be leveled by the cataclysm. If it is a planet, the whole world need not be destroyed; but this has been known to happen.

Like the B-Movies it was usually part of, the "natural" version of this sort of ending is practically a Forgotten Trope nowadays. The "triggered" instance, on the other hand...

Compare Trash the Set. If it happens specifically because the Villain has just died, you have a Load-Bearing Boss.

As an Ending Trope, Spoilers ahead may be unmarked. Beware.

Examples of Cataclysm Climax include:

Film

Lost Continent
The Incredible Petrified World
King Dinosaur (just one island, though)
Indiana Jones and the Last Crusade - Dr. Elsa Schneider tries to take the holy grail past "The Great Seal," which causes the entire temple to fall apart with a local earthquake. This sets up a Take My Hand moment where she tries to reach for the grail while hanging onto Indiana. She can't quite touch it and experiences a Death by Materialism. Indiana nearly does the same but decides to let it go.
- Justified in Indiana Jones and The Kingdom of the Crystal Skull
- Indiana Jones and the Fate of Atlantis. What is it with Indy and Lost Worlds?
In the recent remake of King Kong, this is Skull Island's ultimate fate. Note, however, that this factoid was revealed only on the official promotional website, not in the film itself.
- This idea was taken from the climax of The Son Of Kong, the mostly forgotten sequel to the original version of KK.
Rare example of a deliberately triggered cataclysm: Forbidden Planet, via the planet's Self-Destruct Mechanism.
Another deliberate disaster: the destruction of Metalluna in This Island Earth. Granted, that wasn't the end of the movie, per se, but it fits the trope in every other way.
Beneath the Planet of the Apes. This is what happens when the Lost World is an entire planet. You'd think blowing up the entire world would put a stop to any future sequels. You'd be wrong.
Terminator 3: Rise of the Machines doesn't end with the planet being destroyed, but the massive nuclear launch at the end of the film does effectively end civilization.
In Treasure Planet, the title planet is actually a giant self-destructing explosive device, and the climax involves the main characters attempting to escape the planet before it explodes.
The 2008 version of Journey to the Center of the Earth ends with the area of plant life being burnt to a crisp. It seems that it's cyclical in nature and it may be possible to go back in another 50 years; but with all the magma the flooded the area its hard to imagine how.

Literature

The Mysterious Island ends with Lincoln Island being destroyed by a volcanic eruption.
The first book of the Cambridge Latin Course is set in Pompeii. In the year 79 CE. That the last chapter is a Cataclysm Climax should surprise no one.
T. A. Barron's Heartlight ends with the alien world the heroine visits being swallowed by its sun, which has become a black hole.

Live-Action TV

It appeared for a while that Lost would end like that: starting with a mention of a volcano being present on the Island, then the Island being shown submerged underwater in the Flash Sideways and finally the Man In Black intending to destroy the Island near the end. The Finale appears to play this straight: after the Island's Heart is disturbed, it is shaken by massive earthquakes and several cliffs collapse into the Ocean before the majority of the heroes make their escape. The Trope is then subverted, when the Island's Cork is put back in place (no kidding) and the cataclysm is stopped.
The endings of most dino-documentaries involve the asteriod ending the Mesozoic era.

Video Games

The ending of Uncharted 2: Among Thieves sees the destruction of the Lost City of Shambhala thanks to Nate blowing up the Tree of Life in the final boss battle against Lazarevic.
- Done again in Uncharted 3 When Ubar is swallowed by the sand due to the central pillar being destroyed.
  - With THREE GODDAMNED BULLETS!
In Riven: The Sequel to Myst, you cause this yourself. Deliberately, under orders from Atrus.
Averting this trope is the main objective in Return to Mysterious Island 2: Mina's Fate. In fact, you get the option of averting it twice: once by stopping a volcanic eruption triggered by the previous game's events, and again by choosing to protect the island's ecosystem by restoring the shield Mina deactivated in the first game.