Resonant Moon is a natural satellite, the orbital period of which is in 1:1 resonance with its rotational period, causing there to be a "dark side", which is constantly facing away from the planet. This "dark side" is a perfect site for clandestine facilities.
Increases stealth of buildings on the planet and controlled ships in the same system by 10. The only building requiring this special is Bioterror Projection Base.