The aquifer is the layer of gray in this drawing.
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Aquifer
On Earth, water is held in the ground in a reservoir called an
aquifer. An aquifer is actually a layer of rock which is capable of holding a large amount of water. Some layers are better at holding water than others, for example a layer of sandstone can hold a good deal of water but a layer of shale cannot. The ability of the material to hold water depends upon how fine-grained it is.