Brilliant, scary, thrilling, all good. Only problem was that in solving the mystery, Anna considers all the possible people and omits one. Someone had to get word out that there was someone injured inside. The people that enter were not there before so they were not the ones that come out. (She makes it clear that no radio can get out through these caves.) So the perpetrator could have left the cave never to be seen again. BIG omission, imho, but other than that, this is her best novel to date.