From publisher blurb:
A dying world plagued by ceaseless conflict smolders in the night sky, home to the revelations of Mars.
This sourcebook expands Hollow Earth Expedition to include Mars, a dying and dangerous planet filled with strange aliens, bizarre creatures, and vast, inhospitable wastelands. Fight for survival or vie for supremacy as a marauding sky pirate, powerful robot bodyguard, or conniving Martian noble. Experience Hollow Earth Expedition from an outside perspective — one that wields powerful weapons and hungers for earthly treasures as much as any terrestrial power.
Inside you will find everything you need to run out-of-this-world adventures or give your existing Hollow Earth games a bizarre twist: guidelines for creating robot and alien player characters; new and expanded psychic powers; an unearthly bestiary and equipment list; and details on strange Martian inhabitants and extraterrestrial locations.
Hollow Earth Expedition: Discover a whole new world of adventure!