To understand the signifi cance and role of groundwater in the economy, it is fi rst necessary to understand the existence of groundwater in the ecosystem. Groundwater exists as the subterranean component of the hydrologic cycle. Small organisms that are a part of the biochemical web inhabit the dark, low-oxygen underground environment. Even though we cannot see or observe groundwater easily, it exists in a complex subsurface environment that affects its accessibility and use, its quality, and its sustainability as a resource in the economy. What is the magnitude of groundwater in the ecosystem?