No, Penny didn't enter a Mary Stewart novel. She went into the Crystal Cave under the Naica Mountains in Mexico, a place where you can find huge crystals forty feet long and thicker around than a telephone pole. The place looks like Superman's Fortress of Solitude in the 1978 movie.
Except that the Fortress of Solitude wasn't 130 degrees Fahrenheit, or eighty percent humidity, conditions that prevent any explorers from being able to cool themselves with perspiration. Penny and her fellow explorers had to stuff their clothes with ice in order to cool themselves far enough to last 30 minutes in that hideous environment.
The adventure was documented on the National Geographic Channel. The episode repeats 10pm Eastern time on Tuesday, so check it out.