The interior of an 18" square x 1" piece of Plexiglas was charged to 2.2 million volts (MV) using a 5 MV particle accelerator. A layer of excess electrons become trapped deep inside. When discharged, the excess charge escaped with a bright flash and a loud bang. The hot, lightning-like discharge created thousands of microscopic fractures inside the acrylic, resulting in a branching "Captured Lightning" sculpture (or Lichtenberg Figure).
This video was done by Bert Hickman from Stoneridge Engineering. His website: http://www.teslamania.com