Since long before the advent of supersonic flight, immense booms have been heard emerging from clear skies. No explanation has been found for these so-called sky quakes. There is no sign of lightning (which can travel long distances – even in clear skies) and no other human explanation for why the sky suddenly booms at the earth.
Some believers may consider this evidence for extra-terrestrial activity while sceptics continue to try to find a scientific explanation. Scientists – so far unable to explain the phenomenon – have resorted to explaining it away as one or all of the following: radio waves, solar activity, and even submarine activity (despite the sounds being heard miles inland).