For many centuries, the Earth's water was something of a mystery. Those with an interest in such matters found it difficult to believe that the rather meagre, albeit seemingly inexhaustible supply of rain that fell was anything like sufficient to provide the raw material for lakes and rivers, not to mention keeping the oceans of the world topped up. The concept of some massive reservoir beneath the ground was much preferred, and there were elaborate theories as to how it might be tapped.
Many rather liked a biological analogy. William Caxton, for example, writing in 1480, declared that: "Just as the blood of man runs through the veins of the body and issues forth at a particular place, so water runs through the veins of the Earth and comes to the surface in springs and fountains." And Leonardo da Vinci had very similar ideas: "Water rises to the mountains just like it rises up through plants from the roots to the extremities, as it is seen on vines when they are cut; and as the blood works in all animals, so water does in the world which is a living body."
It was not until the 17th century that it became generally accepted that rainwater was indeed sufficient to cause rivers to flow and aquifers to be replenished. The most respected theorist of the time on these matters was a Frenchman, Edme Mariotte (1620-84), and it was he, too, who first gave a satisfactory explanation as to why, in certain places, fresh water sometimes gushes from the ground in springs.
Springs occur in areas of permeable rock. Some rocks, such as sandstone or limestone, contain tiny gaps or cracks through which the liquid is allowed to pass. In such rock, rainwater can seep down to considerable depths until it is stopped by a layer of impermeable rock below. As more and more water accumulates, it builds up inside the permeable layer until it is saturated like a sponge. Rock saturated in this way is called an aquifer, and the top of the aquifer is called the water table.
Now the water table is normally well below the ground, and its level rises and falls over a given area in a way that roughly corresponds to the ups and downs of the local landscape. Occasionally however, a dip in the topography may cause the water table to be exposed at a particular spot - and there a spring occurs. Of their nature, most springs are in valleys or low places: the water is not defying gravity, but flowing upwards as a result of pressure from water at some higher level in the aquifer.