The thickness of the earth's crust ranges from 50 miles in mountainous areas to just six miles beneath the ocean floor; the average around the world is 21 miles. The borehole, therefore, has already eaten its way through one-third of the earth's surface.
While scientists probe the secrets of outer space, much about the world on which we stand remains mysterious. To expand our knowledge, the Russians started a deep borehole drilling program to investigate geological theories about the formation of the earth.
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