An adequate supply of clean water is essential for life and for health. Water is often drawn from rivers, or may be pumped up out of the ground. Rainwater soaks into the ground and collects in areas where an impervious rocky layer stops the water from draining away. Wells are bored down into this layer to extract the water. A huge mass of fresh water in porous rock beneath the Sahara desert could supply all the water needs of North Africa for hundreds of years. Water from wells is usually stored in reservoirs. Before being used it is purified. The water goes to a settling tank where mud and sediment are removed. Chlorine is added to kill any microbes.
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