ELF (Extremely Low Frequency) waves

Extremely Low Frequency (ELF) waves are electromagnetic waves with very long wavelengths and very low frequencies, typically between 3 and 30 Hz. Because of their long wavelengths, they can penetrate deep into water and soil, making them useful for communication with submarines and geological surveys. ELF waves travel slowly and can pass through many materials without much attenuation, but they require large antennas to generate and detect. Due to their low energy and large infrastructure needs, ELF communications are mainly used in specialized military and scientific applications.