When a water-bearing rock readily transmits water to wells and springs, they are called aquifers. Wells can be drilled into the aquifers and water can be pumped out. On Earth, precipitation eventually adds water (recharge) into the porous rock of the aquifer. The rate of recharge is not the same for all aquifers. On Mars, it is still unclear if the aquifer has been recharged through time and to what extent. Sometimes the porous rock layers become tilted in the ground (because of ground movement, for instance). There might be a confining layer of less porous rock both above and below the porous layer. This is an example of a confined aquifer.