Anti-gravity is the iea of creating a place or object that is free from the force of gravity. It does not refer to countering the gravitational force by an opposing force of a different nature, as a helium balloon does; instead, anti-gravity requires that the fundamental causes of the force of gravity be made either not present or not applicable to the place or object through some kind of technological intervention. Anti-gravity is a recurring theme in science fiction, particularly in the context of spacecraft propulsion. The concept was first introduced formally as "Cavorite" in H. G. Wells' The First Men in the Moon, and has been a favorite deus ex machina since that day.