Aquifers are underground areas of rock or material such as gravel, sand, or clay which hold water. This can happen if an otherwise solid mass of rock has pores or holes (like a sponge) that collect water. This groundwater can be extracted by using a well, and it can be used for farming or for local water supply. Aquifers can still become polluted, even though they are underground. Sometimes too much water can be taken out. This can cause salt water to seep into the aquifers, particularly in areas near the coast. Salt water cannot be used for water supply nor irrigation purposes. In addition, leakage from landfills, wastewater treatment plants and other types of pollution can make its way into the groundwater.