How does water enter groundwater reservoirs?
Rain and snow melt can "soak" into the ground. Some soils and rock formations are porous allowing water to move underground.
Dig a hole in the sand near the ocean. Pour a bucket of water into the hole. In a few minuets the water is gone. The water has been able to "soak" into the sand and move downward into the soil.
In a lava cave in Hawaii we observed fresh water dripping down into pools on the cave floor. The heavy rainfall from the tropical climate allowed water to find routes down thru the lava beds into ground below.