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.
World's five longest surveyed caves. 1  Mammoth Cave, Kentucky, US. 2  Sistema Sac Actun/Sistema Dos Ojos, Mexico. 3  Jewel Cave, South Dakota, US. 4  Sistema Ox Bel Ha, Mexico. 5  Optymistychna Cave, Ukraine.