Our environment is permeated by electrical and magnetic alternating waves in the frequency range above the AC voltage of 50 Hz and also in the radio frequency range. Much attention from the public is given to these waves. Through numerous studies and publications about this type of oscillations and waves it is largely known from which sources they occur and which impact they have. However, very little information could be found about electrical and magnetic alternating waves in the frequency range below 50 Hz. The aim of this research is to demonstrate that these signals exist and also to show how the signals look like and where and when they occur. This article gives an overview of the occurrence of these ELF (Extremely Low Frequencies) signals, their specific properties in view of the time domain and in view of the frequency domain and of the possible sources of these waves. Precise knowledge of the structures of the ELF signals allows conclusions about their potential to cause electromagnetic interference in electronic systems. Also other effects in our environment, eg on flora and fauna could be explained.