For a while when I was working for a calibration company one of my jobs was to calibrate the Hypots used to test insulation in devices such as transformers. Some of these devices look like baby van de graaffs and generate 50,000 or so volts. Unlike the spark machines though they also have enough current capability to hurt you.
On job had me doing such a calibration in an old mill building in Vermont. The unit was inside a safety cage down in the sub basement in a small room at the bottom of a spiral stair case. The lighting was dim and it was the sort of place where you expect to hear water dripping.............or not the sort of place that makes one comfortable fooling around with that much voltage. At max power I could hear the ionization sizzle from my connecting wires as their insulation neared breakdown. At that moment, down there in the dark, alone with canned lightning the only thing that came to mind was the movie Ghost, where the black shadows came up into the world of the living.