Jessica Gage crawls forward through the 10-inch-high rock crevice slowly, pulling with her fingertips and elbows and gently pushing with her toes. Eventually, she worms far enough through the squeeze that she can roll out into the open and stand up. Even then, she’s underneath an entire mountain. Ms. Gage started guiding tours at Raccoon Mountain Caverns in 2005 as a daytime supplement to her night job.