Crystal Onyx Cave is a privately run cave/tour in Cave City, Ky not far from Mammoth Cave National Park.  Visit in the off season for a small group which almost turns into a private tour :-)

This beautiful cave is located at: 425 Prewitts Knob Road, Cave City, KY

Camera used for these photos:  Nikon Z7 with Nikkor 24-70mm f/4 S  (Nov 2018).  All shots are handheld, so there's not as much depth of field as I'd like since I had to shoot wide open at f/4.