Figure 8-12 The Water Hole This graph shows the background noise level from the sky at various radio and microwave frequencies. The so-called water hole is a range of radio frequencies from about 103 to 104 megahertz (MHz) in which there is little noise and little absorption by Earth’s atmosphere. Some scientists suggest that this noise-free region would be well suited for interstellar communication. Within this region dubbed the water hole, the principal source of noise is the afterglow of the Big Bang, called the cosmic microwave background. To put this graph in perspective, a frequency of 100 MHz corresponds to 100 on an FM radio, and 103 MHz is a frequency used for various types of radar.