How deep does phreatic water exist? This depends upon the porosity of the rock, or its ability to contain water. Most mines that penetrate many hundreds of feet encounter little water at great depths. Pressure of the overlying rocks is so great that open spaces capable of holding water cannot exist. So the earth’s crust contains available well water only in restricted zones not far from the surface. The top of the zone of saturation is called the _water table_. It is here that the most rapid flow of phreatic water occurs, for the joints and interspaces are wider nearest the surface. Between the water table and the surface is the _vadose zone_, in which the spaces are partly filled with water and partly filled with air. (See illustrations on page 7). Vadose water content varies greatly with weather conditions. As rainfall is scant in southwest Oregon during the summer, visitors find the caves relatively dry at that time. In the winter, however, the passages will be veritably “raining” vadose water within a few days after snow or rain. The water table itself is more stable, but varies somewhat from winter to summer, or during extended periods of unusually wet or dry seasons. Its lowest possible level is ultimately controlled by the elevation of the largest nearby surface stream or lake, which acts as a base level. When the streams and lakes are lowered by erosion, the water table of a given locality keeps pace by slowly sinking until eventually it lies scarcely above sea level. [Illustration: Solution Rills in Ghost Room ceiling] [Illustration: Crack widened by solution—Watson’s Grotto] Rain falling on the mountains above the cave seeps into the surface cover of vegetation and humus. Here it absorbs carbon dioxide released from the process of organic decay. Seeping further through the vadose zone and down to the water table, this water carries many times the normal amount of carbon dioxide found in the atmosphere. In fact, it becomes acid. For water (H₂O) and carbon dioxide (CO₂) unite to form a mild solution of _carbonic acid_ (H₂CO₃). In this manner, phreatic water is constantly charged with mild acids. Not the kind that harm us, of course. The fountain water by the chalet, and probably that in your home, is actually mild carbonic acid. So is bottled pop. [Illustration: ] _SOLUTION_ RAIN + CARBON DIOXIDE = MILD CARBONIC ACID MOLECULES (H₂O) (CO₂) (H₂CO₃) CARBONIC ACID + CALCIUM = CALCIUM BICARBONATE SOLUTION CARBONATE MOLECULES FROM MARBLE STRATA (H₂CO₃) (CaCO₃) (CaH₂(CO₃)₂) _DEPOSITION_