Probably the atmospheric version of SOFAR; thermal layers in the atmosphere acting as a wave guide for very low frequency sound waves. In the ocean, the sound

Probably the atmospheric version of SOFAR; thermal layers in the atmosphere acting as a wave guide for very low frequency sound waves. In the ocean, the sound of explosions can travel thousands of miles in the SOFAR channel. https://en.wikipedia.org/wiki/SOFAR_channel https://en.wikipedia.org/wiki/SOFAR_channel