Cinematographically, caves are a challenge. So are Nazi survival stories. Set the latter in the former and you've got a double-decker challenge — not impossible, but neither, alas, is it one to which Janet Tobias' well-intended documentary can rise. The film begins with a friendly New York-based spelunker who got in touch with his European heritage by exploring a Ukrainian cave, where he was quite surprised to find human artifacts that had been down there for 70 years. Subsequent inquiries acquainted him with the true tale, also recounted in Esther Stermer's 1975 memoir, We Fight to Survive, of two Jewish families who hid from the Nazis by staying literally underground for nearly a year and a half. Tobias doesn't skimp on the many innately inspiring details of these people's resolve and resiliency, but her uninspired storytelling — vague voiceover narration, dull and too-dark reenactments — makes it hard to connect. The film rightly tries for a climax with footage of the spelunker returning to the cave accompanied by some survivors and their grandkids, but by then all narrative momentum has stalled, and the moment lingers limply. But humanity keeps producing stories like this, so we owe it to ourselves to tell them. Recommended instead, therefore, is Agnieszka Holland's 2011 drama In Darkness, also true, of Poles who took refuge in the sewers.