The area at that time is supposed to have been either an inland sea or a shallow extension of the ocean. During this Permian period the earth's surface was changing. Mountains were rising and the waters were receding, thus greatly enlarging the land areas. Amphibian life was on the wane, and reptiles began to appear. Great thicknesses of limestone deposits were made during these countless centuries, some in the form of a reef now known as the Capitan limestone. Contemporaneous rock behind the reef is called the Tansill formation. It is in these rocks that the Carlsbad Caverns are located. Since Permian time geologists think the seas may have risen again and covered what is now New Mexico, leaving sediments that have been largely removed. About 60,000,000 years ago, during the Cretaceous period when coal was being formed elsewhere in the world and dinosaurs roamed the earth, the land was uplifted, perhaps producing cracks and crevices in the limestone. Geologists at one time believed the Caverns were caused by the action of water as it flowed down through the limestone, dissolving as it did so minute particles of the stone. Today, however, that theory has been discarded, since a more careful and detailed study indicates the Caverns to have been formed by phreatic solution, their development resulting from a two-cycle method of creation. The Caverns can be said to be a natural cavity in the earth formed by the solution of rock by subsurface waters. Actually, there are two great geological processes involved in the formation of the Caverns. The first came about as the water hollowed out the underground chambers, and the second took place when the formations of stone were created in these underground openings. To be explicit, we can only say that the Caverns are large crevices or cracks in the limestone which have been enormously enlarged by the constant solution of the rock into the underground water which filled these indentations. In order to understand just what the two-cycle method means and how it can exist, we must first understand the two conditions under which these operations of nature can take place. Water, as it seeps downward into the earth, tends to seek what we shall call its own level. This would be a point where everything below is saturated with water, the water table of the region. Above the table water works downward due to its gravitational pull. Below the table there is only rather slow movement of water. The area above this point is known as the vadose zone, and below this point the area is the ground-water or phreatic zone.