http://www.csmonitor.com/Science/2015/0402/Why-underground-moon-cities-are-slightly-less-implausible-now?utm_source=Sailthru&utm_medium=email&utm_term=Science_final&utm_campaign=20150402_Newsletter%3AScience_Sailthru&cmpid=ema%3Anws%3AScience%2520Weekly%2520%2804%2F02%2F2015%29