WELCOME TO CARLSBAD CAVERNS
'(<a href="http://www.j-archive.com/media/2008-03-04_DJ_24.jpg" target="_blank">Jon of the Clue Crew wriggles through a small cave opening at Carlsbad Caverns Nat'l Park, New Mexico.</a>)  Carlsbad is part of an ancient one of these structures, on the edge of an inland sea; it was mostly formed by the remains of algae & sponges, not coral'
a reef