" There are many kinds of signals in this extremely low frequency range. Some of them seem to have a natural origin while others resemble communication signals. Independently of their properties some of these signals can be measured in a wide area (30 km circle), others just some meters around a certain location. Also, the temporal appearance is very different. Some signals could be detected over years and then suddenly disappeare for some time. Others were only measured on one day for a few hours and then never again."
Last modified on 21-Mar-17