Doomsday Events:

A doomsday event is a specific occurrence which has an exceptionally destructive effect on the human race. The final outcomes of doomsday events may range from a major disruption of human civilization, to the extinction of human life, to the destruction of the planet Earth, to the annihilation of the entire universe.

A 2006 poll by SciFi.com revealed that virtually all Americans believed that some sort of doomsday scenario could realistically impact the human race, and that many feel that such a scenario is likely to be man-made.

The following are the probable doomsday events:

Natural occurrences:

• A global pandemic
• A geological event such as massive flood basalt(the result of a giant volcanic eruption), volcanism, or the eruption of a supervolcano. One such event, the Toba Eruption, occurred in Indonesia about 71,500 years ago. According to the Toba catastrophe theory, the event may have reduced human populations to only a few tens of thousands of individuals.
• A gamma ray burst or other devastating blast of cosmic radiation. One especially deadly hypothesized source is a hypernova, produced when a hypergiant star explodes and then collapses, sending vast amounts of radiation sweeping across hundreds or even thousands of lightyears of space. Hypernovae have never been observed; however, a hypernova may have been the cause of the Ordovician-Silurian extinction events. The nearest hypergiant is Eta Carinae, approximately 8000 lightyears distant.
• An abrupt reorientation of Earth's axis of rotation.
• A drastic increase or decrease in the Sun's energy output
• An impact event caused by the collision of a large meteoroid, asteroid, or comet with Earth. A common theory postulates that the extinction of the dinosaurs occurred approximately 65 million years ago as a result of the Cretaceous-Tertiary extinction event when a large asteroid struck the earth, producing atmospheric dust which blocked solar energy and caused a significant lowering of temperatures worldwide. Evidence for this theory includes a sedimentary layer of iridium in the geological record and a large crater in the area of Chicxulub, Mexico.
• A sudden change in the physical constants governing the universe, such as that created by a Vacuum metastability event
• The effect of a black hole on the planet Earth.
• An extreme tropical cyclone
• A universal Big Rip or Big Crunch ("Gnab Gib")
  

Non-natural occurrences:

• The creation of a black hole on or close to Earth
• A nuclear or biological war
• A cybernetic revolt
• A grey goo inundation
• Alien invasion
  

Supernatural occurrences:

• An act of divine retribution or the Last Judgment. Many religions include beliefs pertaining to the end of time.
• The Norsemen believed that the world would end in a tremendous battle of the gods known as Ragnarok.
• In the Christian Bible, the Book of Revelation describes Armageddon, a final battle between the forces of God and the forces of Satan.
• In Hindu mythology, Lord Vishnu will assume his tenth incarnation called Kalki Avatar to destroy the world.

In recent human history, the Tunguska event (1908), the Spanish Flu pandemic (1918-19), the atomic bombings of Hiroshima and Nagasaki (1945) and the Chernobyl disaster (1986) stand out as stark reminders that larger-scale events of these kinds are real possibilities.