Europa

"'If life exists beyond Earth in our solar system, it could very well exist on Europa. Slightly smaller than Earth's Moon and blanketed by sheets of ice many kilometers thick, it is theorized that Europa also has a sub-surface ocean of liquid water, warmed by the tidal forces of Jupiter's gravity.""As a water world, Europa will pose unique challenges to the first settlers who land there. It will be up to you to determine whether to leave it as-is and live on top of the ice, or to melt it and risk flooding the whole world.'"