Water that falls to the Earth seeps into the ground. Once underground, it collects in aquifers, which are underground layers of permeable rock that absorb the groundwater. Permeable means that the water can enter into the rock layer and be absorbed like a sponge. The reason that this rock is permeable is because of its porosity, or the amount of pores contained in the rock. These pores are empty spaces contained in the rock that can fill up with water. The more pores there are in the rock (the higher the porosity), the more groundwater the aquifer can hold. These aquifers are important because this is what underground wells tap into to pump water from the ground.