Incomparable Carlsbad Caverns, in southeastern New Mexico, includes a single underground chamber so expansive that its floor could accommodate fourteen football fields and its ceiling could hold a 22-story building; other chambers that contain countless cave formations of great variety of shape and color; cool and naturally circulating fresh air; and a system of lighting that reveals the beauty and spaciousness most effectively. Since the temperature within the Caverns remains at about 56°F the year round, warm clothing is needed. Comfortable shoes, too, for the four-hour, three-mile complete tour, which starts at the natural entrance. Shorter trips, the Big Room tours that start at the elevators in the visitor center, take in only a part of the underground chambers. All tours are under leadership of competent park guides who answer questions and explain the earth processes that have resulted in the caverns and their amazing decorations. From the natural entrance, the immense main corridor of the Caverns is followed downward 829 feet for one and three-quarters miles. This brings visitors to the most scenic rooms (the Green Lake Room, King’s Palace, Queen’s Chamber, and Papoose Room), where the stalactites, stalagmites, and helictites reach their peak in numbers, shapes, and delicate coloring. The trail leads upward 80 feet from the Papoose Room to the lunchroom. Near the lunchroom is the Big Room, the most majestic of the Caverns’ chambers. The trail around its perimeter, one and a quarter miles long, encompasses a floor space of fourteen acres. [Illustration: Hall of Giants in Carlsbad Caverns National Park] Completing the circuit of the Big Room and returning to the lunchroom, visitors may either walk or board an elevator and ride smoothly back to the surface. How were the Caverns formed? The story began about 240 million years ago, during the Permian Period. At that time, the two limestone formations in which the caverns occur—the Tansill Formation and Capitan Limestone—were deposited as part of an organic reef complex at the edge of a warm shallow sea. During subsequent periods, other seas brought in sedimentary material that covered the reef. About 60 million years ago, earth movements, which were responsible for the uplift of the area, fractured the reef and permitted surrounding ground water to enter along fracture lines and begin work in fashioning the caverns. The water at first dissolved small crevices in the limestone. As more water came in, the crevices enlarged to cavities, called solution pockets. Then the walls, floors, and ceilings of the pockets dissolved and collapsed, joining the pockets, while the solution process continued, eventually forming the huge rooms seen today.