Question: how far can a person dig?
Answer: The Kola Superdeep Borehole was just 9 inches in diameter, but at 40,230 feet (12,262 meters) reigns as the deepest hole. It took almost 20 years to reach that 7.5-mile depth—only half the distance or less to the mantle. Among the more interesting discoveries: microscopic plankton fossils found at four miles down.