摘要：If you drill a hole into the ground,in most places its bottom fills with water after a few days.The water appears even if no rain falls and no streams flow nearby.The water that seeps into the hole is part of the vast reservoir of subterranean ground water that saturates the Earth’s crust in a zone between a few meters and a few kilometers below the surface. Ground water is exploited by digging wells and pumping the water to the surface.It provides drinking water for more than half of the population of North America and is a major source of water for irrigation

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