For those who seek the underground, the Southwest hosts the Kartchner Caverns, Colossal Caves, Cooper & Silver mines.  The most recent of all discoveries has been the Kartchner Caverns discovered in 1974 by two amateur cavers from Tucson.

They kept there discovery secret for 14 years to protect it from vandalism & exploitation & to ensure the survival of its delicate ecosystem. Unlike the Colossal Caves, the Kartchner Caverns is a wet, living cave into which water still percolates from the surface above &  calcium carbonate features are still growing.  It has an unusually wide variety of  brilliantly colored cave formations, including the longest  known Soda Straw stalactite formation in the world.

The contrast between the cave's natural 99%+ relative humidity & the dry desert climate above makes this cave particularly vulnerable.  Un-monitored air exchange could quickly destroy the cave's delicate ecosystem, halting it's growth & diminish the cave's natural beauty.