Lechuguilla Cave (New Mexico, USA)
Credit: Dave Bunnell / Wikipedia

What sets Lechugailla Cave apart from others isn't size but its diverse and pristine formations: shelfstones, calcite columns, cave pearls, soda straws, gypsum-frosted stalactites, and enough stalagmites to make the most jaded geologist swoon. Despite being known for years, Lechuguilla was largely ignored until cavers uncovered extensive passages in 1986. More than 136 miles of underground passages have been discovered since then, but only teams conducting scientific research and exploration are allowed to enter.