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.
Entrances may be created by a valley enlarging and exposing a section of the cavern on the side of a hill, the collapse of a sinkhole, or the intrusion of man's developments into the surface. Limestone caverns are commonly found in karst terrain. Karst comes from the Slav word krs meaning crag or stone.