Element 115 was first mentioned in the 1980s in association with UFO conspiracy theories. The most popular account of element 115 is from Bob Lazar, a scientist who claims to have worked on a top-secret, alien craft reverse-engineering program at a site known as S-4 near Groom Lake, Nevada. Many popular culture websites exist which discuss and critique the claims surrounding the element and the field.
Bob Lazar claims that Element 115 exists as a stable isotope. It's used as a fuel via nuclear reactions with proton projectiles in which antiprotons are created. These are then collected and channeled into an annihilation chamber where they react with protons to form gamma rays and lots of energy. This enormous power is used to fantastically multiply the minuscule Gravity-A wave also produced by the onboard Element 115 reactor.
Gravity-A is understood to exist only between atomic nuclear particles and is vastly more powerful than the Gravity-B with which we are all familiar from our own everyday experience. Element 115 is unusual in that the Gravity-A effect extends just beyond the perimeter of the Element 115 atom and with very advanced technology can be tapped and amplified and then directed via gravity distortion devices to produce a kind of ''propulsive'' effect for the alien spacecraft. What is supposedly happening is that the craft is creating a large enough space-time distortion along its direction of orientation that it essentially ''falls downhill'' through the artificially generated gravity corridor.