Tesla theorized that it would be possible to transmit electrical energy through the ground with no loss of electricity. He envisioned a worldwide system of transmitters that put electrical energy into the ground. By tuning the energy to a specific frequency, a receiver could access this power. Tesla regarded this as his greatest discovery. For Tesla to conduct his lightning experiments, he needed a large area of open space.
In May of 1899 and early the following year, Tesla undertook his lightning experiments, which he called terrestrial stationary waves, in Colorado Springs, Colorado. The purpose was to make a world wireless power system to transmit power and messages. He told reporters he was experimenting to send wireless telegraphy signals from Pikes Peak to Paris. Tesla didn't give them any more information, and it's not clear whether he attempted the feat. He built a magnifying transmitter similar to a Tesla coil. At that time it was the largest version of a Tesla coil ever built. The structure was 222 feet high with a 52-foot diameter copper ball on top.
Tesla had some success with his experiments, and they were spectacular. He was able to generate an artificial lightning bolt 135 feet long. The area around the lab glowed when some of the experiments were in session. He lighted vacuum bulbs stuck in the ground at a distance of 25 miles without a wire connection. Some of these experiments took a tremendous amount of power and disabled the generator for all the power to Colorado Springs.
After nine months of testing, Tesla's lightning results weren't clear. He kept detailed notes of the experiments, but it isn't clear how he intended to transmit the power. He theorized that when a lightning bolt struck the ground the earth was a conductor that transmitted electricity with no loss. During this time he also explored the idea of transmitting electricity through the ionosphere. Even though he didn't completely answer all the questions, the experiment wasn't a complete loss. In addition to creating massive man-made lightning bolts, Tesla was correct that the earth and upper atmosphere were conductive.