Formed over thousands, even millions of years, caverns are hidden natural wonders filled with gravity -defying formations made through the Earth’s constant shifting.  While there are some 55,500-plus caves in the U.S., few have become major tourist attractions. Some are too dangerous for amateurs while the discovery of others has just begun.  But you don’t have to be a professional spelunker to explore the beauty down below.  Here are some of the most noteworthy caverns in the U.S.