Kartchner Caverns

Discovered in 1974,Kartchner Caverns opened as a Arizona State Park in 1999.  Located south of Tucson , Arizona, this 'living cave' is composed of a variety of unique minerals and formations.  Water percolates from the surface and calcite formations grow continuously, dripping down like icicles (stalactites) and coming up from the floor (stalagmites).   Tours generally run 1 1/2 hours and reservations are recommended as the number of visitors inside of the caverns at one time are restricted. 
Kartchner Caverns is home to some of the most amazing features:

• one of the world's longest soda straw stalactites: 21 feet 3 inches (Throne Room)
• the tallest and most massive column in Arizona, Kubla Khan: 58 feet tall (Throne Room)
• the world's most extensive formation of brushite moonmilk (Big Room)
  the first reported occurrence of “turnip” shields (Big Room)
• the first cave occurrence of “birdsnest” needle quartz formations
  many other unusual formations such as shields, totems, helictites, and rimstone dams