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1 Water underground

Many people have the impression that underground water occupies vast caverns, such as those in the Derbyshire Peak District, flowing from one cavern to another along underground rivers. This is a common misconception: underground caverns are fairly rare, but huge quantities of water exist underground, within rocks. This is because many rocks contain pores, spaces that come in all shapes and sizes. In sediments, and consequently sedimentary rocks, there are often pores between grains (Figure 1) which can be filled with water. There may also be spaces between rock beds or along joints, fractures or fissures (Figure 2) which can also contain water. However, before we look at pores in more detail we will examine how water gets into the rock.

Figure 1
Figure 1 An electron microscope photograph of a sandstone. Pores make up about 30% of the rock's volume. The largest grains are about 0.3 mm long.
Figure 2
Figure 2 Bedding planes, joints, fractures and fissures in a limestone cliff face. Such features may enable water to accumulate in rock strata.
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