Nothing less than human made lightning. The massive fields generated by resonance between pairs of stepped up induced capacitors create potential differences greater than the resistance of air between the coil and a nearby conductor. This allows fractal currents to flow as the air itself conducts and ionizes.Although Tesla coils are largely created for fun by dedicated enthusiasts, they originally had a real purpose in mind. Tesla figured that he could create a wireless electrical grid and went as far as to build a tower on Long Island that would be its first transmitter. The idea was never realized, however in Russia really large scale wireless power networks were actually tried, as can be seen in this list.Vote for your faves.