For thousands of years, human societies have understood water resources to be renewable, replenished by winter snows or spring rains. In fact, as long as water use is confined to the withdrawal of water from surface sources (lakes and streams), then that understanding is relatively accurate. In the modern world, however, the bulk of the water that is used for most purposes--industrial, agricultural, and domestic--is groundwater. Groundwater is water that has accumulated in a zone of varying depth and thickness below the surface. The quantity of groundwater is often the result of hundreds or thousands of years of accumulation from precipitation and, in some areas, melting glaciers. When humans begin to use or "withdraw" this water faster than it is replenished or "recharged," the consequence is a reduction of groundwater resources. Water is capable of being mined, just like petroleum; and just like petroleum, it can become depleted. The overuse of water (in which withdrawal exceeds recharge) is a result of economic expansion that fails to take into account the environmental costs of doing business.