The hidden species

A girls life changes when she finds an abandoned city-beneath the ground. It was about 5000 ft below the ground.There is no one in the city at first but then she finds them and they are dressed in clothes that haven't been seen before.She befriends a boy,Caleb and soon discovers a startling secret. She must protect him and his family at all costs or they are at risk of losing everything they fought to protect.