BASE jumping is the sport of using a parachute to jump from fixed objects. "BASE" is an acronym that stands for the four categories of objects from which one can jump; (B)uilding, (A)ntenna, (S)pan (a bridge, arch or dome), and (E)arth (a cliff or other natural formation). BASE jumping is much more dangerous than skydiving from aircraft and is currently regarded as a fringe extreme sport (less info)