Under the earth's surface, there is water almost everywhere! You can find it beneath hills, deserts, mountains, and plains. Even though there is water everywhere, most of the time this water is not accessible and it's not fresh to use without treatments. It can even be hard to locate it, measure its depth, and describe it. This water can lie hundreds of feet below the earth's surface or it can occur close to the land surface. Water that is usually hundreds of feet below the earth's surface is in areas of the western United States. Water at very shallow depths can be just a couple of hours old but at moderate depths, it could even be 100 years old. Water that is in a huge depth or that's flown a long distance from places of entry can even be several thousand years old.
There are some of the parts of the precipitation that lands on the earth's surface but it infiltrates into the subsurface. The part that infiltrates into the subsurface will continue downward through the soil and will keep it that way until it reaches rock material that is saturated. Water that is in the saturated groundwater system will move slowly and it could even discharge into the ocean, lake, or stream.
There are a few factors that are responsible for the fact that the groundwater exists:
The first factor is gravity. It will pull the water and everything else towards the center of the Earth. It means that the water that is already on the surface will simply try to seep into the ground that's below it.
The second factor is the rock below our feet. Rock's that are below the surface of the earth are bedrocks. If this bedrock consisted of materials that are dense such as solid granite, then the gravity would have a much harder time to pull the water downward. But the earth's bedrock consists of a lot of different types of rock such as limestone, sandstone, and granite. They also vary in the amounts of void spaces in them and that's where groundwater always accumulates. Also, bedrocks can be broken and fractures and that will create an even bigger space that the water will fill. Some of them, such s limestone are simply dissolved by water and that results in large cavities and it fills with the water.
Try to visualize it this way.

Take two sponges and lay one above the other. Then you take water and pour it at the top and the water will seep through the sponge that on the top and it will go to the sponge that's at the bottom. If you stop watering the top sponge it will dry out and all the water will go to the bottom sponge until it dries as well. But if you take a plastic bag and put it in between the sponges then you will create a barrier and when you add the water to the top sponge it will deep downward until it reaches the barrier and it will not be able to go into the second sponge. That water will start to flowing sideways and you will see it coming out at the edges of the sponge.