New Yorker Chris Nicola made a remarkable discovery while in Ukraine, exploring some of the longest horizontal caves in the world. When he came across signs of former human habitation—buttons, an old house key, a woman's dress shoe—nearby villagers told him that during World War II there were rumors of Jewish families escaping the Nazis by hiding in the caves. It took Nicola nine years to uncover the secret that the cave dwellers had kept to themselves, but now they are ready to tell their amazing story in this gripping documentary about the longest uninterrupted underground survival in recorded human history.