You are a US citizen and: 1  You are a first time applicant age 18 or older. 2  OR your most recent U.S. Passport was issued when you were under age 16. 3  OR your previous U.S. passport was issued more than 15 years ago.
Cave formations are called speleothem, from the Greek word spelaion, cave and thema meaning deposit. Almost all of them are made of calcite, the crystal of calcium carbonate. Different minerals and different movements in the water account for an incredible variety of cave formations.
Underground caves and caverns represent some of the most unique, fascinating (and cool) features of the North American deserts. Many varieties of caves occur in all of the deserts of the Southwest, but are most common in limestone mountains where conifers abound. Such is the case with the two of the most famous caves known in the Desert Southwest, Carlsbad Caverns in southern New Mexico and Lehman Caves in eastern Nevada. These limestone caverns are now protected and managed by the National Park Service.