Tesla conducted experiments contributing to the understanding of electromagnetic propagation and the Earth's resonance. It is well documented (from various photos from the time) that he lit hundreds of lamps wirelessly at a distance of up to twenty-five miles (40 km). He transmitted signals several kilometres and lit neon tubes conducting through the ground. He researched ways to utilize the ionosphere to transmit energy wirelessly over long distances. He transmitted extremely low frequencies through the earth and portions of the ionosphere, called the Kennelly-Heaviside layer, in his experiments.