Settlement of the Moon requires access to resources, especially water for life support and the creation of propellant for spacecraft.  In this video, NASA-Ames scientist Rick Elphic describes the polar volatiles sensed by orbital and impact probes, and how scientists are still searching for a consensus explanation of their origin and distribution.  One theory is that a giant polar ice cap existed two billion years ago, and may still be there under several meters of mixed dirt and ice.