When you walk around on the solid 'ground, you would never guess that there is a layer of water below your feet. In some places there is enough of this buried water to make a lake six feet deep or even deeper, We call it the ground water and much of the water we use in our homes and factories comes from these buried supplies.

Springs and wells, underground streams and rivers are gifts to us from the ground water. And the ground water is the vast supply of water stored in the rocks deep below our feet. The story begins in the clouds that float in the breezy sky.
The clouds are made of moisture and sooner or later this moisture comes tumbling down. Some splashes to earth in silver raindrops, some filters down in feathery snowflakes and some comes pelting down in hail stones. The moisture that falls on the ground starts out on a new career. Some of it rushes down the slopes to join the rivers which wander along and finally dump their waters into the sea. Some sinks down into the soil where it is stored as ground water.
More water seeps into the ground when the rain is gentle, or when the snow melts slowly, or whore there are tangles of plant roots in the soil. Some of it may stay near the surface, trapped in the pockets between crumbs of soil. Some will stay in the spongy pockets of deep layers of porous rocks.
The ground is made from layers of all sorts of different rocks. The slaty layers are hard and solid. Sandstone layers are full of pores like frozen sponges. Limestone layers are soft and crumbly. A layer of solid slate acts like the bottom of a basin, for the water cannot seep through it.
A layer of soft sandstone or limestone may be sitting on the 'bottom of the basin and the ground water stays trapped in these softer rocks.
Maybe the layer of soft rock is on a slope and the ground water tends to move downhill. It dissolves and carries away chemicals, eating and nibbling cracks and crevices into the soft layers of buried rock. In time, the moving ground water digs holes which become caves and tunnels which become underground rivers. Most underground. rivers are dug into layers of crumbly rock buried deep below cur feet by moving ground water.
A winding river on the surface seems to be very careless about the path it takes. But a river must flow over a solid floor and follow the slope of the ground. If the river is covered with, say, loose gravel the stream may still follow its old path, now buried below the surface. Some underground rivers are buried streams which were once on the surface.