Kiki Strike Inside the Shadow City by Kristen Miller

Ananka Fishbein’s life was pretty boring, until she meets Kiki Strike; a strange, tiny, white-haired girl, who seems to magically appear and disappear. Kiki and Ananka stumble upon a huge sinkhole that appears to lead into a forgotten underground world hidden beneath the streets of New York. Kiki and Anaka join together with some other girls  recruited for the unique skills from the Girl Scouts to explore the underground city and it’s supposed hidden treasures. Along they way they end up fighting millions of hungry rats, going to battle with an evil princess and basically saving the entire city.