Atlas Obscura community member Catherine Laton warned me. "You're going to be so surprised at how much you'll love South Dakota." She is one of hundreds of readers who flooded my inbox after I announced my quest to visit all 50 states before July 4th. And, she was right, and also not quite right. I was surprised, but not in the way she meant. Fueled by your e-mails and recommendations , I arrived in the Black Hills at the tail end of winter — roads clear, season not yet turned. The hotel had almost no other guests. A handful of sites were shuttered until Memorial Day, waiting for crowds that hadn't arrived yet. Off-season travel: a little lonely, occasionally frustrating, and then suddenly, quietly, revelatory. Because the caves were open. Jewel Cave sits in the southern Black Hills, not far from where Mount Rushmore keeps its vigil and Crazy Horse slowly emerges from the mountain to the southwest. Below is one of the longest caves in the world.…