Soaked rock, not caverns
Where Is Water Underground?
Underground water fills tiny gaps between grains of rock, like water in a sponge.
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Step 1 of 5
Rock is not solid all the way through.
Sandstone and gravel are made of grains with gaps between them. Those pore spaces can be a third of the rock's volume.
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Step 2 of 5
Rain soaks down and fills the gaps.
Water works down through soil until it reaches rock where every pore is already full. Below that line, the rock is completely saturated.
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Step 3 of 5
The top of the saturated zone is the water table.
Dig below it and your hole fills with water. It rises in wet seasons and drops in dry ones — a well going dry means it has fallen too far.
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Step 4 of 5
It moves, but extremely slowly.
Groundwater creeps through pores at perhaps a metre a day, sometimes a metre a year. Water pumped from a deep aquifer may be thousands of years old.
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Step 5 of 5
It is most of the world's usable fresh water.
Rivers and lakes are the visible fraction. Far more liquid fresh water is underground, which is why so much drinking water is pumped up.
Pump an aquifer faster than rain refills it and the ground above can sink permanently. Parts of California have dropped several metres.
The short version
Groundwater fills the tiny pores in rock like a sponge, moves very slowly, and is most of our usable fresh water.
Try it yourself
Pour water into a jar of gravel. It vanishes between the stones, then a water table appears and rises — an aquifer in a jar.
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