Photo: Stoneridge Engineering
Lichtenberg figures are the branching patterns formed by electrical discharges, discovered by 18th Century German physicist Georg Christoph Lichtenberg. He captured the images in dust on charged plates, but in the 20th Century, laboratories used solid blocks of acrylic, such as the one pictured above. "Captured Lightning" was created by shooting five million volts into the acrylic by the art/engineering firm Stoneridge Engineering. More pictures and an exhaustively detailed scientific explanation at the link.
http://220.127.116.11/frames/lichtenbergs.html via The Presurfer