He believed that the Earth had “fluid electrical charges” running beneath its surface, that when interrupted by a series of electrical discharges at repeated set intervals, would generate a limitless power supply by generating immense low-frequency electrical waves.

https://thefifthestate.com.au/energy-lead/energy/nikola-tesla-dreamed-of-free-electricity-what-happened/

In recent experiments I have discovered two novel facts of importance in this connection. One of these facts is that an electric current is generated in a wire extending from the ground to a great height by the axial and probably also by the translatory, movement of the earth. No appreciable current, however, will flow continuously in the wire unless the electricity is allowed to leak out into the air. Its escape is greatly facilitated by providing at the elevated end of the wire a conducting terminal of great surface, with many sharp edges or points. We are thus enabled to get a continuous supply of electrical energy by merely supporting a wire at a great height, but, unfortunately, the amount of electricity which can be so obtained is small.

https://teslauniverse.com/nikola-tesla/articles/nikola-teslas-free-energy-documents