Nikola Tesla makes what he regards as…

Nikola Tesla makes what he regards as his most important discovery—terrestrial stationary waves—after moving to Colorado Springs, Colorado, in May 1899.

By this discovery he proves that the Earth can be used as a conductor and will be as responsive as a tuning fork to electrical vibrations of a certain frequency.

He also lights two hundred incandescent lamps without wires from a distance of twenty-five miles (forty kilometers) and creates man-made lightning, producing flashes measuring one hundred and thirty-five feet (forty-one meters). (Written accounts from the nearby town of Cripple Creek document odd electrical happenings.)

He also experiments with high-voltage power systems in 1899. (His notes on these experiments, translated into English, will not appear until 1976.)

At one time Tesla is certain he has received signals from another planet in his Colorado laboratory (a claim that is met with derision in some scientific journals). 

He will depart for New York at the beginning of 1900.