A doomsday device is a hypothetical construction—usually a weapon, or collection of weapons—which could destroy all life on a planet, particularly Earth, or destroy the planet itself, bringing "doomsday", a term used for the end of planet Earth. Most hypothetical constructions rely on the fact that hydrogen bombs can be made arbitrarily large assuming there are no concerns about delivering them to a target (see Teller–Ulam design) or that they can be "salted" with materials designed to create long-lasting and hazardous fallout (e.g., a cobalt bomb).