Water is always on the move! Sunlight warms the oceans, lakes, and rivers, and some of the water turns into invisible water vapour. This is called evaporation. The vapour rises into the sky, cools down, and falls back to Earth as rain or snow. Much of this water flows into rivers and lakes and eventually returns to the oceans. Only about 2% of Earth’s available fresh water is found in rivers and lakes. Most of the rest—about 98%—is hidden underground in places called aquifers.

Aquifers are like giant underground sponges. They hold water in tiny spaces between rocks and sand, and people use this groundwater for drinking, farming, and other everyday jobs. Near the coast, these underground water stores can stretch beneath the sea floor. Fresh water from the land can even travel out under the ocean! In some places, people have discovered fresh water bubbling up from springs on the sea floor. It sounds a bit like finding a secret underwater fountain.

But there is a problem. If people pump too much fresh water from a coastal aquifer, seawater can move into the empty spaces underground. This is called saltwater intrusion. Once salty seawater gets into an aquifer, it can be very difficult to remove. Imagine trying to turn a giant glass of salty water back into fresh water—it is not an easy job! Rising sea levels can make this problem even worse.

So groundwater is an important part of Earth’s amazing water cycle, even though we cannot usually see it. We need to look after these hidden water supplies by using water carefully and keeping pollution away from rivers, soil, and groundwater. The next time you drink a glass of water, remember: it may have travelled on a spectacular adventure—from the ocean, through the clouds, into the ground, and perhaps even beneath the sea!

There’s a lot more to read about this aspect of the water cycle and this link goes into just enough detail:  Ghyben-Herzberg Principle