When what was supposed to be a simple exploratory journey sweeps Our Heroes down an underground river, they discover a new world in a gigantic cavern far beneath the surface: Inner-Earth. Inhabited by creatures that long ago died out on the surface, ancient lost peoples and tribes that somehow found their way down there, insidious serpent-men, and a group of lost Nazi explorers whose intentions for the place are decidedly more sinister than the heroes', Inner-Earth contains wonders marvelous to behold... and perils deadlier than anything in the sunlit lands. Do the heroes have what it takes to survive the perils of a hollow world?
— a description of the three main caverns of Inner-Earth and how this strange world at the center of the Earth functions, including four maps of the major areas of Inner-Earth
— descriptions of the four main races inhabiting the place (Aztecs, serpent-men, Mongols, and Neanderthals)
— character briefs for about a dozen NPCs, including "generic warriors" of all four races
— one character sheet and four character briefs for the Nazi expedition that's already stuck down there and scheming for ways to get out
— GMing information and tips, including a GM's Vault of secret, GM's-eyes-only information
— character sheets for seven more dinosaurs to accompany the ones already described in The HERO System Bestiary
— The adventure includes 4 maps for use with Campaign Cartographer!
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