You'll often see electrical trees after lightning strikes. You can spot them in sand, in insulation, and even on very unlucky people's bodies. Generally, they form too fast for us to see them grow, which is why this video is so interesting.
Electrical trees, also known as Lichtenberg figures, generally form very quickly. Here's a chance to see one form in slow motion. Watch as cracks creep across an insulator, clearing the path for a flash of electricity. Here we see cracks in insulation grow slowly, building up to a time when electricity can shoot from one end of the figure to the other.
These trees form in insulation when it is loaded up with electrons. Because the material is a good insulator those electrons have trouble moving through it, meaning the entire structure is buzzing with electrons that can't go anywhere. What usually gets them moving, and makes the figure appear in the insulation, is a crack forming in the insulation. (Often, in videos, you'll see people tap on the insulation with a nail to start a crack forming.) The first crack provides flaws in the insulation and a channel for the electrons. Their movement opens up more cracks, and the movement of the electrons burns the lightning pattern into the figure.
Here we see the process going in slow motion, and cracks slowly build up. At the end, we see a current move from one side of the insulator to the other, through the cracks.
[Via Trap Lightning In a Block.]