Apocalyptic and post-apocalyptic fiction is a subgenre of speculative fiction in which the Earth's (or another planet's) civilization is collapsing or has collapsed. The apocalypse event may be climatic, such as runaway climate change; astronomical, such as an impact event; destructive, such as nuclear holocaust or resource depletion; medical, such as a pandemic, whether natural or human-caused; end time, such as the Last Judgment, Second Coming or Ragnarök; or more imaginative, such as a zombie apocalypse, cybernetic revolt, technological singularity, dysgenics or alien invasion.