If you look at the image closely you can see a fracture in the block at the bottom center. This is where the block was punctured to let out the charge and create a fractal pattern. The block is made of Acrylic (PMMA). It is an insulator.
A 5 million volt particle accelerator called a Dynamitron is used to blast charges into the insulating block of acrylic. The charge would just stay in there if the block was left unattended. But a small nail is hammered into the block to create a pathway for the electrons to flow out which creates an intense, miniature “lightning bolt”. Light it up and the result is an incredible fractal pattern frozen in the glass.
- Trap Lightning in a Block – Popular Science