Hypocenter

A hypocenter or hypocentre, also called ground zero or surface zero, is the point on the Earth's surface directly below a nuclear explosion, meteor air burst, or other mid-air explosion. In seismology, the hypocenter of an earthquake is its point of origin below ground; a synonym is the focus of an earthquake.

Generally, the terms ground zero and surface zero are also used in relation to epidemics and other disasters to mark the point of the most severe damage or destruction. The term is distinguished from the term zero point in that the latter can also be located in the air, underground, or underwater.

Nuclear explosions

The term "ground zero" originally referred to the hypocenter of the Trinity test in Jornada del Muerto desert near Socorro, New Mexico, and the atomic bombings of Hiroshima and Nagasaki in Japan. The United States Strategic Bombing Survey of the atomic attacks, released in June 1946, used the term liberally, defining it as:

For convenience, the term 'ground zero' will be used to designate the point on the ground directly beneath the point of detonation, or 'air zero.'

William Laurence, an embedded reporter with the Manhattan Project, reported that "Zero" was "the code name given to the spot chosen for the [Trinity] test" in 1945.

The Oxford English Dictionary, citing the use of the term in a 1946 New York Times report on the destroyed city of Hiroshima, defines ground zero as "that part of the ground situated immediately under an exploding bomb, especially an atomic one."

The term was military slang, used at the Trinity site where the weapon tower for the first nuclear weapon was at "point zero", and moved into general use very shortly after the end of World War II.

At Hiroshima, the hypocenter of the attack was Shima Hospital, approximately 800 ft (240 m) away from the intended aiming point at Aioi Bridge. At Nagasaki, the hypocenter of the attack was above Urakami Valley, approximately 4,200 ft (1,300 m) away from the intended aiming point at the Mitsubishi Steel and Arms Works.

The Pentagon

During the Cold War, the Pentagon (headquarters of United States Department of Defense in Arlington County, Virginia) was an assured target in the event of nuclear war. The open space in the center of the Pentagon became known informally as ground zero.