Define the underground water

Explanation

**Step-by-Step Solution:**

1. **Understanding the Concept**: Underground water refers to water that is found beneath the Earth's surface. It is an important resource for drinking, irrigation, and various industrial processes.

2. **Definition**: Underground water is defined as the water that percolates through the upper layer of the Earth's surface. This means it seeps down through soil and rock layers.

3. **Process of Percolation**: The process of percolation involves water moving through the soil and rock layers, which can occur due to rainfall or other sources of water. As it moves down, it fills the spaces between soil particles and cracks in rocks.

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