Water Found Deep Inside the Moon
I've always been more excited about a Moon colony than a Mars one. 
I mean, compared to Mars, the Moon is ridiculously close.<p>Many of the same benefits Musk seeks (backup for Civilization, excitement) for a fraction of the cost.<p>Low gravity and atmosphere on a dome would be more complicated, but again, compared to Mars, <i>it's right there</i>.<p>It would be a more humble and realistic first step, IMO.