Another interesting post from www.space.com/28894-moon-lava-tubes-underground-cities.html …. no, not a reference to cities already on the moon, hiding in cavities and lava tubes, but the prospect of earthlings colonising the moon and finding somewhere safe to set up camp. Speculation is that theoretically lava tubes could exist on the moon over half a mile in width, big enough to harbour a lunar colony. They are pictured as offering a refuge from cosmic radiation, meteors, and temperature fluctuations. Sounds quite reasonable when you digest the whole story.