By the way, one thing to clarify with Ghost's posts: The worms are an incredible rarity.  Imagine there's only around 100 fully-grown worms, and about three times that in younger, smaller (but still massive compared to a human) worms that travel alone, in an area that's roughly the size of North America.  A mining expedition getting attacked by a worm is incredibly rare, and there's some rudimentary early-warning systems that can help the miners get to safety before one arrives (although whether it was intended to save the miners' lives or to keep the precious crystals from being lost in the confusion of a worm appearance is debatable).<br/><br/>The worms don't seem to move randomly however, and are often attracted to large groups of people (or other animal life), which is why the cities had to start flying in order to avoid them, but small camps of humanoids can still exist on the surface without too much worry.