Originally released as part of Free RPG Day 2008.
From the pamphlet:
Welcome to Stranded in the Hollow Earth, a free demo adventure for Hollow Earth Expedition. This booklet provides all the rules needed to play through the short Hollow Earth adventure. These rules are only a sampling of the full Ubiquity Roleplaying System, and you will need a copy of Hollow Earth Expedition to play more involved games and campaigns...
For now, though, you have everything you need to take you and your friends on a wild pulp adventure filled with Nazis, dinosaurs, and lost civilizations!