Cultures across the world have imagined actual doorways into the land of the dead, and this National Geographic film tracks them from myth into geography. Greek legend puts one entrance at Cape Tenaron, where Hercules is said to have dragged Cerberus up from the underworld, and another in the story of Hades abducting Persephone. Chinese Taoist tradition describes its own portal to the realm of the dead, built into local folklore rather than treated as metaphor. The film moves to Paris, where the tunnels of the Catacombs are framed as a modern stand-in for the same idea, and to Turkmenistan, where a natural gas fire has burned in the desert for decades under the name the Door to Hell. Scientists and folklorists weigh in on why so many unconnected cultures land on the same image of a hole in the earth leading somewhere worse. The question running through it: are these places genuine religious belief, or just humans mapping their fear of death onto real geology and fire.