I have visited several underground caves and caverns back east, in particular Luray in VA and Mammoth in KY, but they pale in comparison to the Cueva de los Cristales which is featured in the November issue of National Geographic. This cave was discovered in 2000, it's 1,000 ft. underground where the mean temperature hovers around 110 degrees Fahrenheit. It's located in northern Mexico near Chihuahua.
My mind can scarcely grasp the enormity of these crystals! Can you see the "tiny" people crawling over them? No, this is not digitally enhanced, nor is it the scene of a sci-fi movie. Those are real, life-sized people! This is too fascinating to behold!
We will never completely plumb the depths of God's mysteries and creations!