Lichtenberg figures are tree-like structures created in light acrylic plastic when a dielectric breakdown happens. In fact it happens in all types of dielectric materials but are only appreciated in light ones. It can be created by high voltage transformers or electron beams such as delivered by medical linear accelerators (Linacs) but do not try to do it at home (it is dangerous) or even in a hospital production machine because it will damage several components through the beam path. But if you have a Linac that is about to be decommissioned you might want to do it. Bellow it is a link where you will find more information about Lichtenberg figures. And here is a link for a video showing some dielectric breakdowns.
using a medical Linac: http://www.sgsmp.ch/lichtenberg.htm
and more here info here: http://205.243.100.155/frames/lichtenbergs.html