Unconfined aquifers generally occur at shallower depths and often feature a more direct hydraulic connection to surface water bodies but also often of smaller areal extent than confined aquifer systems that is more prone to contamination from land use activities. Deeper, confined aquifers may be more expensive to exploit. Two properties are common to all aquifers. First, just like in the case of surface water, gravity is the main force moving groundwater from continents to water courses and oceans.