World War III (WWIII, WW3 or the Third World War) is a hypothetical worldwide military conflict subsequent to World War II. Because of the development and use of nuclear weapons near the end of World War II and their subsequent acquisition and deployment by several countries, it is feared that a third world war could cause the end of human civilization and most or all human life on Earth. A common hypothesis is that a small number of people could survive such an Armageddon, possibly in deep underground blast shelters or away from Earth, such as on the Moon or Mars or in space vehicles. Another major concern is that biological warfare could cause a very large number of casualties, either intentionally or inadvertently by an accidental release of a biological agent, the unexpected mutation of an agent, or its adaptation to other species after use.