Comets explode in the upper atmosphere periodically. The last one of any size was Tunguska. This is probably the explanation for fused stones -- for example those in the walls of Scottish Stone Age forts that Arthur C. Clarke reported on decades ago.
Comets explode in the upper atmosphere periodically. The last one of any size was Tunguska. This is probably the explanation for fused stones -- for example those in the walls of Scottish Stone Age forts that Arthur C. Clarke reported on decades ago.
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