Risk of ecocatastrophe
Abrupt and widespread discontinuities exist in the fossil record of fauna and are considered evidence of widespread mass extinction of species. These low frequency events have been attributed to fluctuations in sea level, reversals of the geomagnetic fields (exposing the earth's surface to lethal radiation), impacts of the earth by very large meteors (putting tons of dust into the atmosphere cutting off photosynthesis) and supernovae (causing catastrophic but temporary climate changes). There are also a range of potential man-made ecocatastrophes, such as triggering an earthquake with a an underground nuclear explosion. In addition, there are other more fantastic possibilities. The sun will expand into a red star engulfing Mercury and Venus and melting lead on the Earth. The moon can fall to earth, a comet, a swarm of meteorites or a black hole could collide with the earth. The earth will eventually loose its atmosphere. A life form might evolve destroying all of humankind.