Ball lightning is an unexplained and potentially dangerous atmospheric electrical phenomenon. The term refers to reports of luminescent, spherical objects that vary from pea-sized to several meters in diameter. Though usually associated with thunderstorms, the phenomenon lasts considerably longer than the split-second flash of a lightning bolt. Two reports from the nineteenth century say that the ball eventually explodes, leaving behind an odor of sulfur. The actual existence of the ball lightning phenomena is not proven, but they appear in a variety of accounts over the centuries. Until the 1960s, most scientists treated reports of ball lightning skeptically, despite numerous accounts from around the world. Laboratory experiments can produce effects that are visually similar to reports of ball lightning, but how these relate to the natural phenomenon remains unclear.