A spring is a place where groundwater comes to the surface. The earth's crust can have many layers of rock, some porous and some impervious to water. When a layer of impervious rock lies underneath a porous layer, water that has soaked in to the porous part will flow, underground, along the top of the impervious layer. If the level of the ground drops so that the impervious layer is exposed, this water will come out of the ground as a spring.

There are several kinds of springs. Sometimes water is expelled from the earth under pressure, because of the shape of rock layers and channels; in that case it's called an artesian spring. Geysers are another kind of spring. Hot springs occur when groundwater is heated by geothermal activity before coming to the surface. Temperature of the Earth's hot springs varies a great deal -- anywhere from lukewarm to life-threateningly boiling.