Doomsday scenario

A doomsday scenario is a story of how we might meet our horrible end as a species, or even how the Earth might be destroyed. These range from the mythical, to the silly, to the paranoid, to the scientific.

Mythical

 * Armageddon, as described in the Book of Revelation in the Bible.
 * 2012 apocalypse
 * Earth changes.
 * Ragnarok, the Norse mythology ending where (spoiler) Odin dies.

Silly

 * The alien overlords finally tire of anally probing us and vape the planet.
 * Intentional human extinction. (see: Voluntary Human Extinction Movement)
 * The Gays create an evil gay-turning robot that turns everyone GAY!
 * The colors of the rainbow grow so pretty in the sky they grain into the faces of all the people going by.
 * Someone will divide by zero. Luckily most calculators have safeguards against this and will display an error if you try it.
 * The Vogons actually do destroy earth for a Galactic freeway.
 * A planet-eating devours the Earth.
 * Priests in some lost Himalayan country complete writing down the 10,000 names of God.
 * An Imperial fleet that was just about to begin an Exterminatus in an God Emperorforsaken planet is warped in both space and time and does it by mistake on present Earth.

Paranoid

 * The New World Order takes over, and finally tires of anally probing us and vapes the planet.
 * A "Camp of the Saints" scenario, from the novel "Camp of the Saints" by Jean Raspail, in which civilization collapses due to hordes of illegal immigrants.

Scientific

 * Total existence failure, every atom in existence deletes itself (scientific progress goes "boink").
 * Newly discovered more stable form of ice converts all liquid water into ice.
 * Numerous "how the universe may end" scenarios speculated on in popular science and science woo circles.

Astronomical

 * A large asteroid surprises us and impacts the Earth, totally screwing up Archer Daniels Midland's monopoly on genetically modified soybeans.
 * A black hole drops in on the Solar System (immensely unlikely).
 * The Sun becomes a red giant (no need to hurry).
 * Quantum fluctuations in gravity suck the Earth into the Sun (extremely unlikely).
 * Gamma ray bursts (absurdly unlikely).
 * Collision of the Milky Way with another galaxy. Our next impact is scheduled in about 4,000,000,000 years (certain to occur, though the solar system itself will probably be unaffected).

Biological

 * A new virus/bacteria/parasite evolves and eliminates mankind (not exactly doomsday because life will go on. Also has to get around the fact that there exists an inverse relationship between ease of transfer and death rates percentage.)
 * Zombie Holocaust! (less likely).
 * Mass insanity: every person in the world goes insane (halfway there).
 * Someone genetically engineers a virus that kills us all, either deliberately or carelessly.
 * Ecosystem collapse, acid rain or some other environmental disaster destroys the biosphere.
 * Overpopulation: the sheer demand for resources launches a mass global war.
 * Evolution takes over, and humans become a new species. The human race would be "extinct", but our descendants would still be here.

Planetary

 * Peak oil.
 * All life is reduced to grey goo due to nanotechnology.
 * Global thermonuclear war (frighteningly plausible, though less likely to happen on a global scale since the end of the Cold War).
 * Runaway global warming on a Venus-like scale (unlikely; it was repudiated by the same scientist who popularized it. )
 * The Ozone layer peaces out, turning the Sun into a deadly ultraviolet lazer. (Narrowly averted, but but there's a possibility that global warming will cause this anyway. )
 * Yellowstone National Park blows its top (the entire park lies on a supervolcano that has gone off several times in the past, but is very unlikely to erupt again in the near future) [insert fart joke here]
 * Magnetic field reversal (there is nothing in the fossil record to indicate that any previous reversals have caused any problems).
 * Tectonics: the entire Pacific Ocean slides under Asia, sucking the California coast along with it. The Hawaiian hotspot burns through and creates a massive volcano in Siberia. Europe crashes into the east coast of North America. The Himalayas fall over due to being too tall, wrecking most of India. Antarctica breaks into five sections, and each section proceeds north to destroy another continent. See also Expanding Earth Hypothesis.