Exile Game Studio launched their Revelations of Mars, a Hollow Earth Expedition book, Kickstarter campaign.
From the campaign:
This sourcebook expands Hollow Earth Expedition to include Mars, a dying world 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 otherworldly 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 terrestrial games a unique twist: robot and alien player characters; new psychic powers; an exotic bestiary and equipment list; and details on Martian inhabitants and locations